Influence of meteorological parameters on the distribution of precipitation across central Colorado mountains

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THE INFLUENCE OF METEOROLOGICAL PARAMETERS
ON THE DISTRIBUTION OF PRECIPITATION
ACROSS CENTRAL COLORADO MOUNTAINS
by
Lawrence M. Hjermstad
This report was prepared with support from
the National Science Foundation
Grant No. GA-11574
Principal Investigator~ Lewis O. Grant
Department of Atmospheric Science
Colorado State University
Fort Collins~ Colorado
May 1970
Atmospheric Science Paper 163
ABSTRACT OF THESIS
This paper presents the results of an investigation of
meteorological factors causing variations in the distribu­tion
of mountain precipitation with respect to elevation.
Precipitation over both north-south and east-west ridges
along the Continental Divide in the Central Colorado Rockies
has been analyzed to identify the local and general contri­butions
of topography to orographic precipitation.
Across an east-west profile over the Central Colorado
Rockies there is an average of 5.83 times as much precipita­tion
observed at the crest (10,600 feet msl) than at an
average western slope base (5,000 feet msl). All of the
significant increase in west slope precipitation with
respect to increased elevation occurs between 7,000-10,600
feet msl (Avg. 7.49 inches per 1,000 feet).
A maximum precipitation ratio of 1:9.5 (base to crest)
occurs when the 500 rob conditions are west to northwest
airflow greater than 25 mps and temperatures colder than
-30°C. A minimum Frecipitation ratio of 1:3.5 (base to
crest) occurs when the 500 mh airflow is parallel to the
ridge with velocities less than 15 mps and temperatures
warmer than -20°C.
An upper-level low pressure trough and associated
surface cyclonic storm systems are generally located on the
western side of the Continental Divide when relatively
larger precipitation observed at the lower
elevations on the western slope. The upper-level trough
and associated surface storm system are generally located
on the eastern side of the Continental Divide when relatively
high precipitation amounts are observed near the crest of
the ridge (10,600 feet msl) or on the eastern slopes.
Lawrence M. Hjermstad
Department of Atmospheric Science
Colorado State University
Fort Collins, Colorado
May 1970
iv
ACKNOWLEDGEMENTS
The author wishes to express his appreciation to
Professor Lewis O. Grant for his helpful suggestions and
encouragement. An expression of appreciation is also due
Dr. Paul W. Mielke, Jr., Dr. Herbert Riehl and Mr. Charles
Chappell for their useful comments and ideas.
This research was sponsored by the National Science
Foundation under Contract GA-11574.
This material is based upon a thesis submitted as
partial fulfillment of the requirements for the Master of
Science Degree at Colorado State University.
v
TABLE OF CONTENTS
LIST OF TABLES • • viii
LIST OF FIGURES x
INTRODUCTION • . . . • . • • •
Background . • •
Statement of Objective .•.
Literature Review of Mountain
Precipitation Studies .•••.
1
1
3
4
DESCRIPTION OF AREA AND CLIMATOLOGY
Central Colorado Rockies . • • • . • .
Fremont Pass .••..•.•....•••
Vail Pass ••..••.•..•
77
9
11
Relationship between Fremont and Vail
Pas se s. • • • • . . . .. • . • . . . • • • •
Climatology of the Vail-Fremont Pass
Are a . . . . . . . . . . . . . . .
Selection of Other East-West Profile Data
Site s . . . . . . . . . . . . . . . . . . .
Selection of Other North-South Profile Data
Sites . • .
12
13
15
18
PROCEDURE . . . . . . . . . . •
Selection of Precipitation Observations from
Fremont and Vail Passes for Analysis •••
22
22
DATA SOURCES AND REDUCTION TECHNIQUES
Precipitation Data Sources
Precipitation Data Reduction •.••
Meteorological Data
29
29
30
32
ANALYSIS . . . . . . . . • • • . . • . • . . •• 34
The Average Distribution of Precipitation
by Elevation Across the Colorado Rockies 34
Analysis of Resulting Preci~itation
Distribution as a Function o~ 500 rob Wind
Direction . . . . . . . . . . . . . . 37
45
Wind
Analysis of Resulting Precipitation
Distributions As a Function of 500 rob
Ve loci ty . . . . . . . . . . . . . .
Analysis of Resulting Precipitation
Distribution as a Function of 500 rob
Tempe r a ture . . • . • . • . • . . . •. 52
Analysis of the Orographic Precipitation
Profile Across the Colorado Rockies • • •• 57
TABLE OF CONTENTS (Continued)
Synoptic weather Patterns Associated
with Specific Broad-Scale Precipitation
Distribution • • • • • • • • • . • . • •• 58
Synoptic weather Patterns Associated
with Specific Local Precipitation
Pat terns Over Vail Pass • • • . • •• 65
SUMMARY . • •
LITERATURE CITED
. . . . . . .
. . . . . . . . . . . . .
69
71
APPENDIX II •
APPENDIX I . . . . . . . . .
. . . . . .
72
76
vii
Table
1
LIST OF TABLES
Climatology of Freemont Pass Area • 14
2 Vail Climatology . . . . . . 14
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
East-West Profile Stations
North-South Profile Stations
Average Precipitation and Ratios Across
the East-West Profile • . . • . • • • •
East-West Profile Precipitation Amounts in
Inches Stratified by 500 mb Wind Direction ••
North-South Profile Precipitation Amounts
in Inches Stratified by 500 rob Wind
Direction . . . . . . . . . . . . . . . .
Summary of Statistical Tests on Data From
Table 6 . . . . . . . . . . . . . . .
Summary of Statistical Tests on Data
From Tables 6 and 7 . • • • . . • . •
East-West Profile Precipitation Amounts in
Inches Stratified by 500 rob Wind Velocity
North-South Profile Precipitation Amounts in
Inches Stratified by 500 rob Wind Velocity
Summary of Statistical Tests on Data from
Table 10 .
Summary of Statistical Tests on Data from
Tables 10 and 11 . • • . . • • . • • . •
East-West Profile Precipitation Amounts in
Inches Stratified by 500 rob Temperature •
North-South Profile Precipitation Amounts in
Inches Stratified by 500 rob Temperature •
Cases Across the East-West Profile with
Relatively Large Low Elevation Precipita-tion
Amoun.ts . . . . . . . .
Cases Across the East-West Profile With
Only High Elevation Precipitation . . •
viii
19
21
35
38
39
42
45
46
47
50
51
53
54
61
62
Table
18
LIST OF TABLES (Continued)
Synoptic Conditions Associated with High
Precipitation Amounts Observed on West
or East Slope of Vail Pass • • • • • • . • 68
Figure
1
2
3
4
5-a
5-b
6-a
6-b
LIST OF FIGURES
Colorado mountain area selected for study •
Vail, Fremont and Hoosier Pass
precipitation networks .•••
East-west profile of mean topography and
mean elevation of observation sites •••
North-south profile of mean topography and
mean elevation of observation sites • •
Frequency distribution of missing snowboard
observations for each site on Fremont Pass.
Frequency distribution of missing snowboard
observations for each site on Vail Pass ••
Frequency distribution of total number of
missing snowboard observations on Fremont
Pass for a precipitation day•••••••.
Frequency distribution of total number of
missing snowboard observations on Vail
Pass for a precipitation day.•...•••
x
8
10
17
20
25
26
27
28
INTRODUCTION
Background
Colorado State University designed and activated a
research program in 1960 to study the feasibility of
increasing winter precipitation through weather modification
in the Central Colorado Rockies. One of the analysis re­quirements
for this study was the development of a high
density precipitation network crossing the Continental
Divide in several places in central Colorado.
Nearly ten years of winter precipitation measurements
in the target areas under both modified and natural precipi­tation
occurrences are now available for study. Although
most of the analysis to date have been concerned with
comparisons of modified and natural precipitation amounts,
this analysis has generated a need to better understand
the precipitation distribution with regard to elevation
under differing precipitation episodes. This is the major
objective of this study.
The precipitation that occurs in the mountainous
regions results from three condensation mechanisms acting
individually or in combination. First, there is the
synoptic horizontal convergence of the air mass into the
low pressure center of the storm which causes ascending
motion~ The precipitation resulting from this mechanism
is, in general, uniformly distributed along the storm
track under non-changing storm conditions.
2
The second precipitation mechanism is the forced
lifting of the air mass caused by the increase in elevation
of the topography. This orographic lifting results in a
second distribution of precipitation that is directly re­lated
to the rate of elevation change of the underlying
topography. The contribution to orographic precipitation
diminishes and reverses as soon as the mountain crest has
been crossed.
The orographic contribution of precipitation may be
divided into a broad-scale lifting caused by the mean ele­vation
change across the mountainous region and a local
orographic effect caused by sharp local elevation change
in the topography. Depending upon the orientation of these
elevation changes to the air flow, these local effects can
playa dominant role in the precipitation distribution
across small segments of the storm track as it moves
across the mountains.
The third contributing mechanism to mountain
precipitation is convection. Convective precipitation is
generally less discernable in winter storms in the Rockies
and is most noticeable in intensifying storm systems or
fast-moving cold fronts with sharp temperature discontinu­ities.
Furman (1967) describes the presence of convective
motions in the form of banded structures embedded in the
orographic snow clouds of Colorado. With only scant radar
coverage and partial histories of recording gage measure­ments
of precipitation for the data analyzed in this paper,
3
no attempt was made to isolate the convective from the
orographic and convergence types of precipitation.
statement of Objective
In order to identify and analyze various precipitation
distributions with elevation, a good understanding of the
parameters influencing natural precipitation distributions
is needed. The objectives of this study are as follows:
1. To describe the average distribution of
precipitation with elevation across the
Colorado Rockies.
2. To determine the changes in the Central
Colorado mountain precipitation profiles
caused by variations in 500 rob meteoro­logical
parameters.
3. To describe the synoptic meteorological
conditions producing general and local
variations in the distribution of pre­cipitation
across the Colorado Rockies.
Once the average distributions of precipitation over
the Colorado Rockies have been identified, a basis for
analyzing or comparing other precipitation distributions
can be established. Precipitation patterns over other
mountainous regions can be compared to those over the
Colorado Rockies in an attempt to study their geographic
variations. Also, natural and modified precipitation
distributions can be compared over the same mountain to
better understand the influence of weather modification on
precipitation patterns.
4
Literature Review of Mountain Precipitation Studies
A study of the precipitation regimes over the Upper
Colorado River Basin (MarIatt and Riehl, 1963) has shown the
role the large and small storm occurrences play in producing
runoff into the Colorado River. Another study of precipita­tion
as a function of elevation was made over the Colorado
and Wyoming mountains using wintertime precipitation
(Finklin, 1967) by interpolating between precipitation
sites. The distances between precipitation sites ranged
from 3 to 36 miles. Elevation changes between compared
sites ranged from 940 to 4200 feet. The annual precipita­tion
increases for the elevation changes that were studied
ranged from 2.14 to 17.36 inches per 1000 feet with an
average change of 6.39 inches per 1000 feet. This large
range of increase in precipitation with elevation resulted
partly from the averaging of precipitation differences over
a large range of distances and from variations in the mean
elevation between the precipitation sites. Also, the
geographical location of some of the compared sites was
such that they were more favorably located with respect to
the climatological storm track through the Rocky Mountains.
A much higher average change of 11.4 inches per 1000
feet was calculated by Finklin (1967) in a study over the
Sierra Nevada Mountains. This higher average rate of
increase in wintertime precipitation with increase in
elevation is mainly due to the higher moisture content of
the warmer low altitude type clouds moving over the Sierras.
5
For the regions between Eagle, Colorado, and Vail Pass
and that between Leadville, Colorado, and Fremont Pass,
Finklin calculated wintertime precipitation increase changes
with elevation of 3.51 inches per 1000 feet and 6.37 inches
per 1000 feet respectively.
In all the above studies, only general inferences were
made that meteorological parameters affected the distribu­tion
of precipitation with increased elevation.
A study in the Southern California mountains by Elliott
and Shaffer (1962) showed some of the meteorological and
dynamic factors that contribute to the formation of oro­graphic
precipitation. Their study also presented a
theoretical distribution of this precipitation across the
mountain range once it had been condensed from the model
cloud system.
Peck and Brown (1962) and others have developed tech­niques
for preparing isohyetal maps for mountain regions.
From these maps correlations were derived between precipita­tion
amounts and physiographic features. From their work
they calculated increases of precipitation with respect to
elevation to be between 2.3 to 4.6 inches per 1000 feet for
several mountainous regions of Utah. No attempt was made
to identify the meteorological parameters contributing to
the terrain's influence in the distribution of precipita­tion
in this study.
Williams and Peck (1962) investigated the terrain
influences on precipitation distribution over the mountain
6
regions of Utah relative to various general synoptic
weather patterns. They found that under upper-level cold
law situations the rate of increase of precipitation with
elevation was approximately 34% of the rate of non-cold
law situations. The rate of precipitation increase with
elevation is .99 inches per 1000 feet for cold laws, and
the rate for non-cold laws is 3.33 inches per 1000 feet.
The studies mentioned have discussed the variation of
mountain precipitation with elevation as a function of a
specific synoptic pattern, physical properties, and dynamics
of the storm system. But, how do the variations in meteorol­ogical
parameters change the pattern of orographic precipi­tation?
This study of the precipitation patterns resulting
from changes in various meterological parameters will lead
to a better understanding of mountain precipitation.
7
DESCRIPTION OF AREA AND CLIMATOLOGY
Central Colorado Rockies
The mountainous region in Colorado considered in this
investigation is shown in Figure 1. The region consists
roughly of the northern two-thirds of Colorado. The western
half of the region is made up of flat-top mountains with
peaks up to approximately 9000 feet msl along the western
edge of the state. The mountain tops gradually become
more rugged and increase in elevation to the Continental
Divide in central Colorado. Here the peaks reach over
14,000 feet msl. The elevation in the eastern half of the
region drops abruptly from the Divide to a relatively flat
elevation around 5000 feet msl.
The mountainous region to the west is interrupted by
three main river drainages: the Yampa River Valley to the
north, the mainstem of the Colorado River Valley in the
central portion, and the, Gunnison River Valley to the south.
Each of these river valleys is oriented roughly west-east
up to the Continental Divide which is generally oriented
north-south. One of the effects resulting from the orienta­tion
of these valleys is the channeling of the airflow to
the Continental Divide where the air rises more abruptly
and the distribution of precipitation with elevation change
becomes most pronounced under favorable synoptic conditions.
Figure 1. Colorado mountain area selected for study.
R
9
Fremont Pass
Fremont Pass lies on the Continental Divide at an
elevation of 11,318 feet. It is approximately 25 miles east
of the Sawatch Range and at the western foot of the Mosquito
and Tenmile Ranges all of which rise to heights over 14,000
feet.
The orientation of the Arkansas River Valley from
Leadville to the pass is approximately 210-230 degrees and
from the pass to Frisco the Tenmile Creek Valley is oriented
approximately 360-10 degrees.
The limited source of the moist air for orographic
precipitation from the southwest comes mainly from air
moving up the Arkansas River Valley south of Leadville or
from moist air spilling over from the Gunnison River Valley.
The source of moisture from the north is mainly from air
moving up the Blue River Valley from the North Park region.
Figure 2 shows the precipitation network from Leadville
across Fremont Pass to Frisco. The network consists of
24 snawboard sites spaced about one mile apart along the
highway and two recording precipitation gages. In addition,
precipitation data taken from a snowboard and recording
gage site at the University of Colorado's High Altitude
Observatory supplements the Colorado State University net­work
data and aids in determining the timing of the
precipitation episodes.
Vail, Fremont and Hoosier Pass precipitation networks
I-' o
r (,
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Figure 2.
1\ 1\
11
Vail Pass
Vail Pass is located approximately 10 miles north of
Fremont Pass and the Continental Divide which runs east­west
through this section of the mountains. The Pass is in
the western river drainage and is at an elevation of 10,603
feet. It is approximately 20 miles east and slightly north
of the Sawatch Range and at the western foot of the Gore
Range.
The orientation of the western half of the pass is
mainly from 310-330 degrees and follows the Gore Creek
drainage. A small segment of the lower precipitation net-work
has an orientation more towards 250-270 degrees but
has a less pronounced topographic change. The orographic
effect from an easterly flow was not considered in this
analysis. Easterly winds below 13,000 feet are nearly
impossible over Vail Pass due to the location of the
Tenmile Range which lies about 10 miles east of the Pass.
The source of moist air for orographic precipitation
from the west and northwest results from an airflow up the
Colorado River Valley into the Eagle River Valley and also
from the moist flow moving up the Yampa River Valley into
the Upper Colorado and Eagle River Valleys.
Figure 2 also shows the precipitation network from
Minturn across Vail Pass to the junction of Highways 6 and
91 south of Frisco. The network consists of 21 snowboard
sites spaced approximately one mile apart along Highway 6.
There were no recording precipitation gages in this network
11......-
12
during the period for which data are included in this
study. Because of the relative closeness of the Vail
precipi tation network to the recording gage at HAO and the
recording gage on the north side of Fremont Pass, a good
estimate of the timing of precipitation episodes was always
obtained.
Relationship between Fremont and Vail Passes
As was stated in the preceding section, Vail and
Fremont Passes are located approximately 10 miles apart
(straight-line distance). This closeness and their dis­similarity
in orientation presents an opportunity to
evaluate the local orographic effects on the precipitation
distribution across these two passes under identical
synoptic conditions.
The average distance across the storm precipitation
tracts that move through the Rockies is approximately 200
to 600 miles. With the distance twenty to sixty times
larger than the distance separating the two passes, it can
accurately be assumed that the general synoptic conditions
producing precipitation in the Colorado mountains are
nearly identical over both passes.
The main features then that should cause variations in
the precipitation distribution across these passes should
be the orography and orientation of the passes relative to
the synoptic flow of moist air. With the mean orientation
of the two passes approximately perpendicular to each other,
a mean air flaw parallel to the orientation of one of the
13
passes would be favorable for inducing additional orographic
precipitation. The same flaw should be less favorable for
the generation of additional orographic precipitation for
the other pass. Therefore, under simultaneous precipita­tion
episodes, a comparative analysis is available to define
the local topography effect of a single mountain barrier in
producing additional orographic precipitation.
Climatology of the Vail-Fremont Pass Area
The winter synoptic weather pattern is dominated by a
long wave ridge positioned over the western United States
which produces fair weather over the Rocky Mountains. This
pattern is often disturbed by large traveling cyclones which
move over the mountains leaving behind important amounts
of precipitation in the form of snow. From Table 1 it can
be seen that the majority of the annual precipitation
associated with the cyclones occurs over this area in the
winter months. During the summer months afternoon convec­tive
showers occur almost daily and account for most of
the summer precipitation.
Table 1 lists climatological data for Climax and
Sugarloaf Reservoir, Colorado, from United States Weather
Bureaus' Colorado Hourly Precipitation, data for the
period 1957 - 1967 and includes all precipitation events
during the period studied. Grant et ale (1965) reported
that approximately 70% of the total precipitation falls in
this area at intensities of 0.03 inches per hour or less
~
Days wi th Days wi th Total
Days of .10 inches .50 inches Precipitation
Location Precipitation or more or more 203 Days
[HAO] Snowboard-C1irnax 2NW 203 136 12 39.64 inches
Fremont Summit 203 137 10 36.55 inches
Vail Summit 203 160 20 49.87 inches
*Season is from 1 November to 30 April.
Table 2. Vail Climatology
(Based on 203 specific precipitation days)
I-'
~
17.82 inches
25.90 inches
Mean
annual
precipi tation
9 . 79 inches
Mean
season*
precipitation
4.10 14.12 inches
1.82
Days per
season* with
.50 inches
or more
precipi tation
50.22
33.54
Days per
season* wi th
.10 inches
or more
precipitation
Table 1. Climatology of Freemont Pass Area
(1957 - 1967 -- All precipitation events)
66.18
87.72
Days per
season* with
recorded
Site precipi tation
Climax 2 NW
Sugarloaf
Reservoir
I
15
and that the intensity is 0.01 inches per hour or less for
45% of the time.
No long record of meteorological data exists for Vail
Pass. However, Table 2 includes a crude climatology using
the average from the three highest precipitation sites at
Vail Pass for a comparison with the HAO snowboard near
Climax on Fremont Pass and the average of the four highest
sites on Fremont Pass. Identical dates of snowboard
observations for natural precipitation episodes are com­pared.
Relative to the Climax site and the top sites on
Fremont Pass, Vail Pass receives slightly more precipita­tion.
A possible explanation is that Vail Pass has a nearly
open exposure to the westerly moist airflow while Fremont
Pass is partially sheltered by the Sawatch Range and
consequently less moisture is available for orographic
precipitation.
Selection of Other East-West Profile Data Sites
Up and down wind precipitation sites were selected in
a generally east-west direction from Vail Pass in order to
extend the profile across the entire northern Colorado
mountain region. Only Weather Bureau stations with re­cording
gages were selected. A profile was used starting
at an elevation of 5000 feet. On the western side,
traversing the mountains and Continental Divide and drop­ping
back to an elevation of 5000 feet. The initial and
residual precipitation observed at the lowest elevations
It
16
would presumably be that produced by the synoptic
convergence of the synoptic circulation of the storm system.
Figure 3 shows a mean east-west profile of the
topography and of the average elevation of the precipitation
sites used in this study. The horizontal axis is distance
east and west from the Continental Divide. At each of the
distances marked on Figure 3, a north-south line was drawn
on a topographic map similar to Figure 1. Along each of
these lines a mean elevation was evaluated and graphed on
Figure 3. Figure 3 shows that within 30 miles of the
Continental Divide there is a more rapid rate of increase
in the elevation of the observation sites up to the ridge
line relative to the rate of increase in elevation of the
mean topography. It would be expected that this region
should show the most significant changes in precipitation
amounts due to the orographic effect under various weather
patterns.
For the mean east-west precipitation distribution
analysis, ten western and seven eastern stations were
selected to extend the dense data network over Vai 1 Pass.
These stations are located within 125 miles of Vail Pass
at elevations ranging from 5000 - 9000 feet. Each of the
stations has hourly precipitation records for the period
analyzed. Names of towns in Figure 1 with no marks after
them indicate those stations used for the east-west profile.
.....
-.....J
93 East
Denver
Deertrail
............ ---
Morrison
Wood Lake
Pork
45 57
Evergreen
Elk Creek
Lake George
° Fremont Pass
Vail Pass
38
Eagle, Aspen
Crested BuHe
Gunnison
90 72
Meeker Craig
Rifle Wilcox Ranch
Cedaredoe
Mean Eleva'ion ~ _1.9e.e>9."~~:::n _ _ _ _ _ _ _ _ _ _ _ . s\o\\v,;
Elevation of Obsef'l°'\Ol'
125 West
Grand Junction
6,000
Distance in Miles
c
4,000
Figure 3. East-west profile of mean topography and mean elevation
of observations sites
- 12,000 Q)
~ 10,000
c: 8,000
.Q-o
>~
w
I
18
The seventeen precipitation sites with recording gages
generally lie in the southwest to northwest and southeast to
northeast sectors from Vail Pass. Table 3 lists the sites,
elevation range, station elevation, their approximate
straight-line distances from the Continental Divide and the
corresponding 24-hour precipitation period used in this
analysis. In designing the profile network in this fashion,
most of the sites are affected by the same weather systems
that produce precipitation on Vail Pass. Usually more than
one station was available for determining the precipitation
at each elevation range as is shown in Table 3.
Selection of Other North-South Profile Data Sites
Five sites with recording precipitation gages were
used in extending the north-south precipitation distribution
analysis beyond Fremont Pass. This analysis was undertaken
mainly to look at the variations in the precipitation dis­tribution
across Fremont Pass under northerly airflow
conditions. The stations were located within 80 miles of
Fremont Pass and each station had precipitation records for
the period analyzed. The stations used in the north-south
profile are indicated in Figure 1 with a small triangle
after the name of the town.
Figure 4 shows a mean north-south profile of the
topography extending from either side of Fremont Pass and
the average elevation of the observing sites. The hori­zontal
distance is again distance from the Fremont Pass
area, and the mean elevation was evaluated from a
Table 3. East-West Profile Stations
19
(Comparative 24-hour Precipitation Period is from 0900-0800
hours. )
38
38
47
39
50
40
7-9
3-5
0-1
3-6
72
90
91
70
60
46
125
81
70
110
35
16-19
Distance to
Continental
Divide in
Miles
6,000
6,285
6,242
6,175
5,960
7,760
7,300
4,855
5,400
8,430
8,500
5,221
5,183
6,497
7,664
Station
Elevation
in
Feet
8,855
8,949-9,125
7,928
7,872-8,176
9,718-10,232
10,488-10,626
10,200- 9,839
5,000
5,000
8,000
8,000
8,500
8,500
5,000
5,000
7,500
7,500
6,000
6,000
6,000
6,000
7,000
7,000
9,000
9,000
6,000
Elevation
Range
in
Feet
10,000
10,500
10,000
Craig
Meeker
Cedaredge
Wilcox Ranch
Grand Junction
Rifle 2 ENG
Station
Eagle
Gunnison
Morrision 1 SW
Aspen
Vail #79,78,77
Woodlake Park 8 NNW
Evergreen 25W
Denver WBAP
Deartrai1
Elk Creek
Lake George 8 SW
Vail #69,68,67
Vail #65,64
Vail #62A,62,61
Crested Butte
Vail #72,71
4,000
~
o
~ ...... _-
47 51 Nath
Hot SUlphur Grand Lake
Springs
15
Frisco
10 °
Sugarloaf Fremont
Reservoir Pass
Distance in Miles
68
Coaldale
\-IIea~ ~\;.';: !!..o~ ..'!f_T..'!P..'!~~~ _ _ _ . -S-la-li"-'" -
lI1eo ! Q'oSe"al\Oo
n Eleva"ot' 0
87 South
Saguache
Figure 4. North-south profile of mean topography and mean elevation
of observation sites
c
8,000
12,000
co
; 6,000
>
Q)
w
+-
~ 10,000
lL.
21
topographi cal map simi lar to the method used in the
profile.
A glance at both Figures 3 and 4 show that the highest
mean elevation in the northern two-thirds of Colorado is
located in the vicinity of Vail and Fremont Passes. This
would suggest that generally orographically generated pre-cipitation
should be evident across this area regardless of
airflow direction.
Table 4 lists the station that represents each
elevation and the corresponding 24-hour precipitation
period. Again, since the sites are within 80 miles of
Fremont Pass, the same weather system that produces precipi-tation
at Fremont Pass will effect the whole network.
Table 4. North-South Profile Stations
Station Distance to Comparative 24
Elevation Fremont Hour Precipi-
Station in Feet Pass in Mi les tation Period
Hot Sulphur Springs
2 SW 7,800 47 0900-0800 hours
Grand Lake 6 SSw 8,300 51 0900-0800 hours
Fremont #23,24 Avg. 9,200 18 0900-0800 hours
Fremont #20,20A Avg. 10,000 12 0900-0800 hours
Fremont #11,12,
13,14 Avg.ll,200 2 0900-0800 hours
Fremont #1,2 and
Sugar load Reser-voir
Avg. 10,100 12 0900-0800 hours
Sagauche 7,700 87 0900-0800 hours
Coaldale 6,900 68 0900-0800 hours
22
PROCEDURE
Selection of Precipitation Observations from Fremont and
Vail Passes for Analysis
Precipitation observations from both the Vail and
Fremont Pass networks from the 1960-61 winter season to the
1967-68 winter season have been used in this study. Precip-itation
measurements across either pass were made during
this period if a quarter inch of snow was observed to have
fallen at any of the sites.
Observations were normally made across Fremont Pass
starting with the site nearest Leadville (Fig. 2) at 0800
hours. The precipitation at all the sites was consecutively
measured across the pass to the site nearest Frisco which
was normally observed around 1100 hours. Observations
across Vail Pass started at 0800 hours with the site near-est
Minturn and then were continued consecutively across
the pass to the last site near the junction of U.S. 6 and
Colorado 91. This last site was normally observed around
1030 hours. The observation technique and data reduction
to equivalent inches of water will be explained in the
Data Sources and Reduction techniques section.
Snowfall occurs very infrequently during the period
of the morning when the snowboards are being observed.
Observations and investigations by Grant et ale (Bureau of
Reclamation Report, 1969) indicate that a strong diurnal
occurrence of no precipitation occurs between 0800 hours
and 1100 hours over the Fremont-Vail area. For this reason
.J
23
the timing for each observation across these two passes
eluded were as follows:
not included.
tion events for the winter seasons from 1960 to 1968 are
The feasibility study operated for varying lengths
The data analysed for this study is only part of a
2. More than 9 precipitation sites were missing in
the network observation.
1. Several days of precipitation were included in one
observation.
analysis purposes. Only 24-hour accumulations were used in
In this study only precipitation data from natural
this analysis since it is the objective of this study to
of time each winter season. Precipitation events were
24-hour portion thereof.
unless otherwise indicated. From 1961 to 1968, 329 natural
has been compared to an average timing of 0900 hours for
on precipitation during a single precipitation episode or
precipitation episodes were recorded at Fremont Pass and
24-hour precipitation episodes were considered for analysis
passes, 264 were used from Fremont Pass for this study and
study.
256 from Vail Pass. Reasons for dropping those not in-complete
set of precipitation data used in the National
evaluate the influence that meteorological parameters have
288 were recorded at Vail. Of these occurrences on both
ity study was operating. Consequently, all the precipita-
Science Foundation sponsored weather modification feasibility
seeded by Colorado State University for a 24-hour period
starting at 0900 hours on a random basis when the feasibil-
24
3. More than 4 consecutive sites were missing in the
observation.
At the time of each original precipitation observation, a
comment was marked on the data sheet as to any pecularities
that were noticed in the snowfall observation such as wind-swept
or melted. On the basis of these remarks, sites that
deviated markedly from its neighboring sites were catego-rized
as missing by data analysts, a frequency distribution
of missing observations was made for each site on each pass
for the network observations used in this analysis and are
listed in Figures 5a and 5b.
Three sites on Fremont Pass were dropped because they
were missing more than 25% of the observations. A frequency
distribution of the number of missing sites for each
observation day for each pass is shown in Figures 6a and 6b.
Figure 5-a Frequency distribution of missing snowboard observations
for each site on Fremont Pass
IV
lT1
(19.3)
51 (18.2)
48
Dropped
( 213)
72
(7.2)
(42)(3.8)(3.8) 19
II 10 10 I I I
(13.3)
35
Dropped
(27.6) Dropped
73 (25.8)
68
(19.7)
52
Snowboard Site Numbers
2 3 4 5 6 7 8 9 II 12 13 14 15 16 17 18 19 20 21 22 23 24
20A
(12.1)(11.7) (11.4) (102)
32 31 30 (8.3) 27 (8.7)
(6.8) (7.6) (6./) (7.6)22 I 23
18 20 16 20
(2.3) I(2;:) r I
264 Episodes t Percent of
Total in Parentheses.
(4.2)
"I
80
70
60 ~
~ :v 50 ~
a.
40 ~
30
20
10
a
"0
L. oo
.n
01 ~
.C- c0
~ if)
-o VI
L. C
(1) .Q
.En -0
:) ~ z (1)
VI .n o
Figure 5-b Frequency distribution of missing snowboard observations
for each site on Vail Pass
tv
0'\
(6.3) (5.5)(5.1)
16 (iP I~ 13
( 14.4)
37
(18.7)
48 (16.0l
41
Snowboard Site Numbers
256 Episodes, Percent of
Toto I in Parentheses
61 62 636465 66 67 686970 71 72 73 74 75 76 77 78 79 80
62A
(7.8) I (10.1)
(6.3) 20 26
16
100
'UI- 90
0
0 80 ~
~
01 0 70
C C 'en CJ) .~ I- 60
~ Q) a. 50 -o II) c 40
I- 0
Q) '- .0 ...... 30 E g
:::J I- 20 Z Q)
II)
~ 0 10
0
100 r 95
I
90
80
(/) 70 <l)
(/) o 60 I-
0
'0 50 ~ 46
.... 40 I 35 33
~ 30
I
E 20 z::J 20 f- 16
10 f- II' T I 0 a a
0 I 2 3 4 5 6 7 8 9 10
Number of Missing Snowboard Observations per Precipitation Occurance
Figure 6-a Frequency distribution of total number of missing snowboard
observations on Fremont Pass for a precipitation day
N
-J
46
t 26 IV
21 2.0 co
/3
J 3 a 4 I a I , I
a I 2 3 4 5 6 7 8 9 10
Number of Missing Snowboard Observations per Precipi tation Occurance
130
120 ~
110
100
90
~ 80
en
oo 70
.... 60 o
.... 50 Q) E40
::J
Z 30
·20
10
a
Figure 6-b
122
Frequency distribution of total number of missing snowboard
observations on Vail Pass for a precipitation day
29
DATA SOURCES AND REDUCTION TECHNIQUES
Precipitation Data Sources
The hourly precipitation data for the sites listed in
Tables 3 and 4 and for the climatological study (Tables 1
and 2) was obtained from the Colorado Hourly Precipitation
Data published by the U.S. Department of Commerce. However,
for a more rapid analysis a computer tape of this data was
obtained.
On Vail and Fremont Passes precipitation measurements
were obtained from snowboards using essentially the standard
Weather Bureau procedure for observing snowfall. Compari­sons
of observations from snowboards, shielded and un­shielded
gages made by Grant (1961), show that snowboards
properly exposed will consistently give nearly identical
observations as a shielded precipitation gage. Since all
the sites on Fremont and Vail Passes had been selected
considering exposure, the snowboard network was considered
to have the same accuracy and reliability as an identical
non-recording precipitation gage network.
Recording precipitation gages located along the Fremont
network (Fig. 2) were used to check both the timing of the
precipitation and the amount that was recorded over the
24-hour precipitation period comparable to the snowboard
accumulation period.
Precipitation Data Reduction
The technique for observing the precipitation on a
snowboard consists of making three snowdepth measurements
about 8 inches apart in triangle pattern on the board. An
average depth of snow is then recorded on the daily log.
A core sample of snow is taken with a thin-walled
cylinder midway between the three depth measurement points.
This sample is put in a plastic bag which is then sealed
and marked with the precipitation site number. This sample
is later weighed and the weight of the snow is obtained.
With the weight of the sample, a cross-sectional area of
the cylinder, and density of water known, the equivalent
depth of water can be calculated.
For precipitation sites with missing observations, an
interpolation method was set up to fill in the missing
observations. In analyzing the precipitation data for both
passes, five (three distinct and two less distinct) pre-cipitation
distributions could consistently be observed.
The precipitation distributions for each pass were clas-sified
and grouped into the following five main types:
1. Precipitation amounts of 3 times or greater on
the westerly side of the Pass in comparison to
the amounts on the e'asterly side.
2. A symmetrical distribution of precipitation
amounts across the pass.
3. Precipitation amounts of 3 times or greater on
the easterly side of the Pass in comparison to
the amounts on the westerly side.
31
The less distinct precipitation distributions were as
follows:
4. l~ to 2~ times greater precipitation amounts on
the westerly side of the Pass in comparison to
the amounts on the easterly side.
5. l~ to 2~ times greater precipitation amounts on
the easterly side of the pass in comparison to
the amounts on the westerly side.
With approximately 260 observational days available for
analysis from each pass, a large sample was obtained for
each of the five basic distribution types. The average
amount of precipitation was calculated for each site along
the two precipitation networks for each of the five dis-tributions.
The profile across each site's average preci-pation
amount showed the relationship between neighboring
snawboard precipitation amounts for the five distributions.
With this relationship and also using the next two avail-able
observations on each side of the missing site, a value
for the missing observation was obtained for sites within
a distribution type.
By using this interpolation method to complete the
precipitation observations across each of the passes, a
daily 24-hour accumulation of precipitation for selected
elevations and sites from western to eastern Colorado was
available for analysis.
Precipitation during any 24-hour period frequently
cccurred with changing meteorological conditions. To better
isolate the conditions that produce a specific precipitation
pattern, each epiSOde's hourly precipitation amounts were
32
analyzed as to the type of duration. The episodes that had
70% or more of the total 24-hour precipitation recorded
within 6 hours either side of a standard upper air observing
time (OOOOZ or l200Z) and those episodes whose upper air
conditions were nearly identical for both standard observing
times during the 24-hour period were used as the basis of
the various distribution analysis. Each of the above pre­cipitation
events now has single valued upper air meteorol­ogical
values which can later be grouped for analysis.
More than 75% of the nearly 260 individual 24-hour episodes
for each pass were in the above category. The remaining
sample was analyzed for the unique characteristics that
were evident in the 24-hour upper level meteorological
condition, such as upper air temperature change, wind shift,
etc.
Meteorological Data
To represent the conditions of the atmosphere during
a precipitation episode, the 500 rob level wind data and
temperature were used. Since many of the peaks along the
Continental Divide rise to over 14,000 feet msl and the
effect of the terrain may still be reflected somewhat in
the atmospheric parameters at 16,000 feet, the 500 rob level
at about 18,000 feet was considered to be the most represen­tative
and convenient level depicting the storm's structure
in a free atmosphere.
The Fremont and Vail Pass area lies between Denver and
Grand Junction at a distance ratio of 2 to 3, respectively.
33
A linear interpolation of the 500 rob radiosonde value of
temperature, wind direction and speed between Denver and
Grand Junction for the Fremont-Vail area was calculated
using the above 2 to 3 ratio. Values were interpolated
from both the OOOOZ and l200Z radiosonde observation for
each precipitation episode and if necessary slightly ad­justed
to better fit the isothermal or isobaric wind pat­tern
on the corresponding 500 rob U.S.W.B. Facimile chart.
In analyzing the thermal and wind patterns between Denver
and Grand Junction at 500 robs, the two stations generally
had temperatures within 2°C of each other. The wind
directions and velocities were usually within the range of
30 degrees and 10 mps. The accuracy from this interpola­tion
is about one degree C for temperature and 10 degrees
and 3 mps for the wind direction and speed.
34
ANALYSIS
The Average Distribution of Precipitation by Elevation
Across the Colorado Rockies
An average distribution of precipitation by elevation
for the Colorado Rockies was determined from the 256 pre­cipitation
events across the east to west profile. The
observed precipitation amounts at each elevation increment
were totaled and divided by 256 to determine an average
precipitation amount per 24-hour event. These average
precipitation amounts are listed by elevation increment
in Table 5.
The 256 precipitation events were randomly obtained
over the eight winter seasons. The average precipitation
amounts per elevation increment from these 256 events com-pose
a relationship which is representative of the mean
wintertime precipitation distribution across the Colorado
Rockies over Vail Pass.
The west and east 5,000 feet precipitation average of
.05 inches per 24-hour precipitation event is considered to
be that resulting from the general synoptic convergence of
the storm system. From Table 5 the average low elevation
precipitation of .05 inches was compared to the maximum
high elevation precipitation of .237 inches per 24-hour
event. From this comparison it was calculated that 4.74
times more precipitation is observed near the highest
elevation precipitation sites than at the average 5,000
foot precipitation site. This additional precipitation
Table 5. Average Precipitation and Ratios Across the East-West Profile
(256 cases)
Elevation in feet Total Precipitation Average Precipitation Precipitation Ratio
in inches in inches (Base 5,000 ft.W)
5,000 W 10.280 .040 1.000
6,000 W 13.531 .053 1. 325
7,000 W 8.818 .034 .850
8,000 W 31. 245 .122 3.050 w
(J1
9,000 W 44.108 .172 4.300
10,000 W 60.587 .237 5.925
10,500 59.570 .233 5.825
10,000 E 43.657 .171 4.275
8,500 E 8.350 .031 .775
7,500 E 12.165 .048 1.200
6,000 E 19.056 .074 1.850
5,000 E 15.485 .060 1.500
~
Disregarding the insignificant changes
Most discussions of mountain precipitation are con-
36
over the mountain range results from the forced lifting of
the air mass over this portion of the Rocky Mountains.
This orographic precipitation will be further considered
From U.S. Weather Bureau Hourly Precipitation records,
It is of interest to note in Table 5 that in the mean
later in this analysis section.
there is no significant change in precipitation amounts
between 5,000-7,000 feet msl. Consequently, all the signifi-cerned
with changes in amounts of precipitation with eleva-the
mean 5,000 foot msl western slope wintertime (1 November-tion
or total precipitation profiles. The average
30 April) precipitation amount is 4.62 inches. The base
and 10,600 feet msl.
cant increases of precipitation amounts occur between 7,000
7.49 inches per 1,000 feet.
of precipitation amounts between 5,000-7,000 feet msl, the
for this average profile between 7,000-10,600 feet msl is
ratio is 1:5.-83.
resulting increase in precipitation with respect to elevation
(5,000 feet msl) to crest (10,600 feet msl) precipitation
precipitation profile in Table 5 can be used as a relation-ship
to derive such a distribution of precipitation by
elevation across a north-south oriented mountain for a
winter season. From the derived profile, the changes in
precipitation amounts between specific elevations can be
calculated.
[
37
A list of the ratios of the 5,000 foot west slope
precipitation amount to each of the other precipitation
amounts at the stated elevations is given in Table 5. This
list of ratios gives the reader a base upon which he can
better picture the increases in precipitation amounts with
elevation. The mean relationship of the 5,000 foot west
slope precipitation ratio over the east-west profile from
Table 5 and several actual winter precipitation totals will
give the necessary multiplication factors to complete the
estimated profile. An example of this interpolation is
given in Appendix I.
Analysis of Resulting Precipitation Distribution as a
Function of 500 fib Wind Direction
In analyzing the various precipitation patterns across
the mountain range from west to east and from north to south,
the available cases were stratified by 500 rob wind direction,
wind velocity and temperatures during each percipitation
episode.
Tables 6 and 7 respectively, list the average precipa-tation
amounts per 24-hour precipitation event for various
elevations by different wind directions for the west to east
and north to south precipitation profile networks. From
Table 6 it can be noted that as the wind direction becomes
more west to northwesterly, the amount of precipitation re-ceived
at the upper precipitation sites above 9,000 feet
increases by about 50% and the "precipitation shadow" to
the lee of the Continental Divide becomes more pronounced.
11:..... _
.053
.040
256
Average
.035
.057
33
Special
Episodes
.041
.050
11
.032
74
.027
81
.047
.069 .034
.092
4 50
.079
.082
3
.017
.027
Table 6. East-West Profile Precipitation Amounts in Inches Stratified
by 500 mb Wind Direction
No.
of
Cases
Range -> 30°-60° 150°-190° 200°-240° 250°-290° 300°-340° 350°-20°
6,000 W
Elevation
in feet 4­5
,00° ~'V
7,000 W .020 .016 .047 .045 .024 .016 .022 .034
8,000 W
9,000 W
.119
.083
.080
.115
.123
.178
.129
.182
.132
.186
.098
.121
.093
.142
.122
.172
w
00
10,000 W
10,500
.141
.133
.164
.172
.209
.189
.239
.228
.288
.304
.172
.165
.196
.190
.237
.233
10,000 E .170 .111 .147 .162 .228 .129 .118 .171
8,500 E
7,500 E
6,000 E
5,000 E
.143
.246
.327
.252
.110
.157
.172
.194
.051
.051
.060
.062
.015
.032
.058
.034
.041
.065
.114
.090
.024
.028
.052
.076
.013
.017
.021
.017
.031
.048
.074
.060
Note: There were zero number of cases in the 70°-140° range.
Table 7. North-South Profile Precipitation Amounts in Inches Stratified
by 500 rob Wind Direction
No.
of
Cases 2 4 52 90 82 10 24 264
Range -> 30 0 -60 0 150 0 -190 0 200 0 -240 0 250 0 -290 0 300 0 -340 0 350 0 -20 0 Special Episodes Average
Elevation
7,800 N -t .050 .092 .079 .059 .045 .043 .030 .056
8,300 N .130 .060 .081 .078 .052 .052 .048 .067
9,200 N .008 .102 .123 .136 .138 .066 .093 .126
10,000 N .025 .095 .152 .166 .202 .140 .140 .169
w
11,200 .059 .137 .156 .177 .201 .170 .144 .176 '-0
10,100 S .043 .120 .108 .105 .111 .041 .061 .101
7,700 S .035 .145 .049 .013 .020 .002 .005 .023
6,900 S .140 .017 .036 .014 .004 .006 .010 .016
Note: There were zero number of cases in the 70 0 -140 0 range.
In a
/1 I !
J' /
classl.cal
40
500 rob trough associated with surface
cyclones, the main precipitation associated with the trough
is on the leading edge where southwesterly winds exist. As
the surface cyclone and upper level trough passes an area
and the upper level winds shift direction from southwest to
northwest the precipitation observed at the lower elevations
diminishes. This decrease in precipitation results as the
dynamic influence of the synoptic convergence diminishes.
At higher elevations the orientation of the topography to
the wind's direction still can be influential in the con-tinuation
and intensity of orographic precipitation.
In Table 6 it can be observed that the maximum low
elevation average precipitation is associated with south to
southwest wind directions and it diminishes as the winds
shift to the northwest. At the highest elevations the
average precipitation increases as the winds shift from a
southerly direction to northwesterly direction. This im-plies
that orography influences the generation of mountain
precipitation to a greater extent than the dynamic con-vergence
of the synoptic storm.
The ratio of the 5,000 foot precipitation to the maxi-mum
amount received near or at the ridge line increases from
1:3.0 for south-southwest wind directions to 1:11.3 for
northwest wind directions. The low and high elevation pre-cipitation
amounts used to calculate these two ratios and the
ratios were tested several ways by using the sum of squared
ranks test with adjustments for tied observations (Mielke,
1
1967). A summary of the test results with P-values is
shown in Table 8. TheP-values are the probability of
observing a more extreme test statistic under the null
hypothesis than the one observed. The tests show that the
low elevation precipitation amounts decrease significantly
and the :high elevation amounts increase significantly as the
wind direction shifts from a southwest direction to a north-west
direction. The decrease in low elevation precipitation
amounts appears to be strongly associated with the high
occurrence of zero amounts under a northwesterly 500 rob
wind direction.
As a result of this analysis, it can be stated that
the wind direction plays a major role in the distribution
of precipitation received above 9,000 feet on the central
Colorado Rockies. Average increases up to 50% from a non-orographic
south-southwesterly direction to an orographic
northwesterly direction can be expected above 9,000 feet
msl.
The average precipitation at the low elevations east
of the mountains increases as the wind direction becomes
more orographically oriented with an easterly component.
The southeasterly wind directions are usually associated
with closed cold low patterns evident at 500 rob approaching
or passing to the south of the analysis region. The east-erly
wind direction with this weather pattern normally
extends down to the surface and produces precipitation
along the east side of the Rockies.
J
42
Table 8. Summary of Statistical Tests on Data
From Tab Ie 6
Test
Data Tested Statistic
Occurrence of zero precipita­tion
at low elevations under
S.W. flow vs occurrence of zero
precipitation at low elevations
under N.W. flow 4.85
Maximum high elevation amounts
when zero precipitation at low
elevation for S.W. flow vs
N•w. flow 1. 0 0 3
Ratios of low elevation preci­pitation
to maximum high eleva­tion
precipitation for all non­zero
low elevation cases S.W.
flaw vs N.W. flow 3.833
All low elevation precipita­tion
amounts for S.W. flow
vs N. W. flow 4 • 86 0
All maximum high elevation
precipitation amounts for S.W.
flow vs N.W. flow 2.541
Degrees
of
Freedom P-value
122 .03%
52 23.1%
68 .04%
122 .004%
122 .65%
The northeasterly wind directions are usually
associated with strong surface high pressure systems that
are pushed up along the east side of the Rockies. Oro-graphic
precipitation from the associated easterly wind
flow at the surface results along the east side of the
Continental Divide. This flow does not normally extend
much beyond the Continental Divide.
In general, distinctly different weather pattern is
needed to produce precipitation on the west side of the
Continental Divide as compared to the weather pattern
-
43
necessary for precipi tation on the east side of the Rocky
Mountains.
The orographic effect is less pronounced across the
north-south profile at all elevations. Although the
orientation of Fremont Pass is essentially southwest to
north-northeast, it should be recalled that the pass lies
just to the western side and nearly parallel to the
Mosquito and Tenmile Ranges which rise to over 14,000 feet
ms 1 and are oriented north-south. These ranges should be
more influential in the generation of a more general oro­graphic
precipitation over this area than the local effect
resulting from the orientation of Fremont Pass.
Forty-one precipitation cases common to both Tables
6 and 7 were studied in evaluating the general orographic
precipitation under a southwesterly 500 rob wind direction.
The average 5,000 foot precipitation amount per case from
Tab Ie 6 for these cases was ,.064 inches. The average high­er
elevation maximum was .155 inches for a southwesterly
direction. The resulting low to high elevation ratio was
1:2.4.
Sixty-one precipitation cases were similarly evaluated
from Tables 6 and 7 for a northwesterly 500 rob wind direc­tion.
The resulting ratio of the average 5,000 foot amount
of .031 inches to the average high elevation amount of .228
inches was 1:7.4.
The high and low elevation precipitation amounts and
ratios were tested similar to the data for the east-west
I~I
44
profile using the squared ranks test. A summary of the test
results are shown in Table 9. Again the results show that
both the low elevation precipitation decreases and maximum
high elevation precipitation increases significantly as the
wind direction shifts from southwest to northwest. The
decrease in the low elevation precipitation amounts again
appear to be strongly associated with the high occurrence
of zero amounts under a northwesterly 500 rob wind direction.
For the more general orographic precipitation over the
central Colorado Rockies, it can be stated that wind direc­tion
strongly influences the average increase in precipita­tion
above 9,500 feet msl. This increase approaches about
30% when the wind direction becomes perpendicular to the
surrounding mountain ranges.
The local "lee effect" of precipitation dis tribution
for northerly wind directions is well pronounced over the
south side of Fremont Pass by only traces of precipitation
being recorded at the elevations around 7,000 to 8,000 feet
rosl. Also, the southerly wind direction indicates sizable
increases in precipitation on the south side of the pass
between 8,000 to 10,000 feet msl implying same local effect
of the orientation of the profile.
The effect of the orientation of Fremont Pass appears
to be less significant on the overall distribution patterns
of precipitation than was apparent across Vail Pass. The
general orographic effect of the nearby mountains appears
to be the controlling influence on the precipitation pat­terns
observed over Fremont Pass.
45
wind velocities, from about 50 to 80% from velocities less
than 7 mps to those greater than 26 mps.
.05%
.04%
.60%
1.7%
20.2%
P-value
42
56
100
100
100
Degrees
of
Freedom
Test
Data Tested Statistic
Maximum high elevation
amounts when zero precipi­tation
of low elevations
for S.W. flow vs N.W. flow 1.115
Occurrence of zero precipi­tation
at low elevations for
S.W. flow vs N.W. flow 3.950
Table 9. Summary of Statistical Tests on Data
From Tables 6 and 7
various 500 rob wind velocity ranges for the east to west
All low elevation precipita­tion
amounts forS.W. flow
vs N.W. flow 3.945
and north to south precipitation distributions respectively.
amount per 24-hour precipitation event for an elevation for
Ratio of low elevation
precipitation to maximum high
elevation precipitation for
all non-zero low elevation
cases for S.W. flow vs N.W.
flow 2.640
Not much vari ation is 'noted in the precipi tation
amounts in each velocity group below 7,000 feet in both
tables. At the higher elevations a distinct trend is noted
Analysis of Resulting Precipitation Distributions As a
Function of 500 rob Wind Velocity
Tables 10 and 11 show the average precipitation
All maximum high elevation
precipitation amounts for
S.W. flow vs N.W. flow 2.185
in both tables for higher precipitation amounts with greater
Table 10. East-West Profile Precipitation Amounts in Inches Stratified by
500 rob Wind Velocity
No.
of
Cases 20 77 81 45 33 256
Range-> 0-6mps 7-15 16-25 >25mps Special Episodes Average
Elevation-l-
5,000 W .046 .049 .039 .030 .035 .040
6,000 W .062 .056 .050 .047 .057 .053
7,000 W .041 .029 .037 .046 .022 .034
8,000 W .086 .109 .145 .141 .093 .122 ~
0'\
9,000 W .123 .141 .198 .225 .142 .172
10,000 W .199 .193 .265 .307 .196 .237
10,500 .205 .190 .255 .310 .190 .233
10,000 E .134 .139 .196 .233 .118 .171
8,500 E .071 .037 .030 .029 .013 .031
7,500 E .057 .059 .053 .037 .017 .048
6,000 E .066 .077 .095 .076 .021 .074
5,000 E .072 .072 .075 .041 .017 .060
~
Table 11. North-South Profile Precipitation Amounts in Inches
Stratified by 500 rob Wind Velocity
No.
of
Cases 16 79 94 51 24 264
Range -> 0-6mps 7-15 16-25 >25mps Special Episodes Average
Elevation 4-
7,800 N .037 .042 .065 .078 .030 .056
8,300 N .044 .053 .074 .093 .048 .067
9,200 N .127 .092 .129 .188 .093 .126
10,000 N .146 .128 .173 .244 .140 .169 .c:.
-....J
11,200 .142 .143 .174 .256 .144 .176
10,100 S .072 .079 .099 .166 .061 .101
7,700 S .075 .024 .023 .016 .005 .023
6,900 S .022 .021 .019 .001 .010 .016
;::::==
48
The low elevation values on the north-south profile
would not be representative for comparison with the high
elevation sites due to the relationship previously dis­cussed
with the surrounding mountain ranges. It is interest­ing
to note the nearly consistent ratios of from 1:2.7 to
1:3.9 for all specific wind velocities. This consistency
emphasizes the fact that the orientation of Fremont Pass
plays a minor role in altering the precipitation distribu­tion
across it when compared to the general orography of the
surrounding ranges.
In studying the influence of the 500 rob wind velocities
over the north-south profile, precipitation cases cornmon
to both Tables 10 and 11 were used.
Sixteen cases were evaluated for wind velocities less
than 7 mps. The average 5,000 foot precipitation amount
per case from Table 10 for these cases was .036 inches. The
average higher elevation maximum from Table 11 was .146
inches for these light-wind velocities. The resulting low
to high elevation ratio was 1:4.0.
Thirty-eight precipitation cases were similarly
evaluated from Tables 9 and 10 for wind velocities greater
than 25 mps. The resulting ratio of the average 5,000 foot
amount of .033 inches to the average high elevation amount
of .265 inches was 1:8.0.
The high and low elevation precipitation amounts used
to calculate the two ratios in both east-west and north­south
profiles and the ratios were tested using the squared
49
ranks test. A summary of the test results for both profiles
are shown in Tables 12 and 13, respectively. The test
results for both profiles show that the low elevation
amounts are not changing significantly with respect to the
high elevation amounts as the velocities change from less
than 7 mps to greater than 25 mps. The high elevation
increases are statistically more significant on the north­south
profile than on the east-west profile where the
maximum high elevation amounts increase by about 80% for
velocities greater than 25 mps. The high occurrence of zero
precipitation amounts at the lower elevations when wind
velocities were greater than 25 mps appears to be signifi­cant.
This high occurrence of zero precipitation does
significantly alter the basic distribution of the low
elevation precipitation across the low and high velocity
east-west profiles as indicated by that specific test. The
same is not evident for the north-south profile.
From an analysis of the data in Tables 10 and 11, it
can be seen that wind velocity plays an important role in
the distribution of the precipitation across a mountain
range. The higher the wind velocity the stronger will be
the resulting vertical motion. These stronger vertical
motions through the cloud system over the mountain range
produce condensate at a a more rapid rate which precipi­tates
out over the upper windward side of the mountain.
The slight decrease in precipitation with elevation
between 6,000 and 7,000 feet from Table 10 may be partially
50
Table 12. Summary of Statistical Tests on
Data from Table 10
Test
Data Tested Statistic
Occurrence of zero preci­pitation
at low elevations
for velocities less than
7 mps vs velocities greater
than 2S-mps 3.76
Maximum high elevation
amounts of precipitation
when zero precipitation
at law elevations for
velocities <7 mps vs
velocities >25 mps-- 1.505
Ratios of low elevation
precipitation to maximum
high elevation precipita­tion
for all non-zero low
elevation cases for
velocities <7 mps vs
velocities >25 mps-- .072
All law elevation preci­pitation
amounts for
velocities <7 mps vs
velocities >25 mps-- 2.025
All maximum high elevation
precipitation amounts for
velocities <7 mps vs
velocities >25 mps-- 1.204
Degrees
of
Freedom
63
30
31
63
63
P-value
.04%
10.1%
45.8%
2.4%
17.5%
explained by the selection of precipitation sites at 7,000
feet. They are located in the bottoms of canyons and
consequently no vertical motion would result from either
a northwest or southwest wind. Also, there is no signifi-cant
increase in the topography of the downwind canyon to
make the westerly wind more efficient in producing preci-pitation.
51
Table 13. Summary of Statistical Tests on Data
from Tables 10 and 11.
Test
Data Tested Statistic
Occurrence of zero preci­pation
at low elevation
for velocities <7 mps vs
velocities >25 mps 2.020
Maximum high elevation
amounts of precipitation
when zero precipitation at
low elevations for veloci­ties
<7 mps vs velocities
>25 mps. 1.185
Ratios of low elevation
precipitation to maximum
high elevation precipita­tion
for all non zero
low elevation cases for
velocities <7 mps vs
velocities >25 mps-- .620
All low elevation precipi­tation
amounts for
velocities <7 mps vs
velocities >25 mps-- 1.276
All maximum high elevation
precipitation amounts for
velocities <7 mps vs
velocities >25 mps-- 1.730
Degrees
of
Freedom
52
23
27
52
52
P-value
2.4%
18.9%
33.6%
16.7%
4.6%
The large precipitation increase between 8,000 and
10,600 feet can be explained mainly by the fact that this
change in elevation occurs in a fairly short distance of
about 30 miles while the distances between 5,000 to 8,000
feet occur over longer distances of about 90 miles. The
region with the more rapid rate of increase in elevation
has the more rapid rate of increase of precipitation.
52
The stronger wind velocities have associated with them
a more pronounced "precipitation shadow" on the lee side
of the mountain. This "precipitation shadow" results as the
air rapidly moves across the crest of the ridge and decends
on the lee side at a dry adiabatic rate and warms the
environment. This warming is sufficient to evaporate nearly
all the condensate that was available for precipitation.
Analysis of Resulting Precipitation Distribution as a
Function of 500 rob Temperature
Tables 14 and 15 show the average precipitation amount
per 24-hour precipitation event for various elevations and
500 rob temperature ranges for the east to west and north to
south precipitation distributions respectively.
Little change or even a slight decrease in the maximum
high elevation precipitation is observed in the -16°C to
-25°C category when compared to the -21°C to -25°C category
in both Tables 14 and 15. A decreasing trend in the average
precipitation at the higher elevations with decreasing
temperatures colder than the -21°C to -25°C category can
also be noted. From the -21°C to -25°C category to the
-26°C to -30°C category on both tables the maximum higher
level precipitation decreases by 14% and 21%, respectively.
From the -21°C to -25°C to the category colder than -30°C
the decreases are 33% and 37%, respectively. The four cases
in the warmest category do not constitute a large enough
sample from which any sound conclusions may be drawn.
Table 14. East-West Profile Precipitation Amounts in Inches Stratified by
500 mb Temperature
No.
of
Cases 4 39 93 66 21 33 256
Range -> OOC to -15°C -16°C to -20°C -21°C to -25°C -26°C to -30°C <-300C Sp~cia1
--- Eplsodes Average
Elevation-t
in feet
5,000 W .077 .047 .040 .042 .025 .035 .040
6,000 W .099 .053 .056 .052 .025 .057 .053
7,000 W .115 .025 .039 .038 .025 .022 .034 U1
w
8,000 W .197 .129 .139 .108 .111 .093 .122
9,000 W .309 .191 .189 .156 .136 .142 .172
10,000 W .330 .255 .261 .225 .175 .196 .237
10,500 .305 .239 .263 .224 .168 .190 .233
10,000 E .254 .195 .192 .159 .133 .118 .171
8,500 E .006 .034 .035 .0'47 .012 .013 .031
7,500 E .000 .065 .044 .063 .039 .017 .048
6,000 E .000 .123 .072 .083 .064 .021 .074
5,000 E .000 .114 .054 .068 .045 .017 .060
Table 15. North-South Profile Precipitation Amounts in Inches Stratified
by 500 rob Temperature
No.
of
Cases 6 45 106 60 23 24 264
Range ->O°C to -15°C -16°C to -20°C -21°C to -25°C -26°C to -30°C <-30°C Special
Episodes Average
Elevation+
in feet
7,800 N .135 .080 .061 .048 .013 .030 .056
8,300 N .082 .095 .067 .067 .029 .048 .067
9,200 N .168 .149 .134 .121 .080 .093 .126
U1
~
10,000 N .206 .188 .182 .156 .123 .140 .169
11,200 .241 .188 .197 .156 .124 .144 .176
10,100 S .167 .119 .111 .090 .069 .061 .101
7,700 S .053 .040 .022 .015 .028 .005 .023
6,900 S .012 .040 .013 .010 .003 .010 .016
It
55
It should generally be expected that less precipitation
would occur at colder temperatures at any elevation since
the potential condensate decreases with decreasing temper­ature.
As observed on an adiabatic diagram, the potential
liquid water decreases by 60% from -20°C to 30°C.
When comparing the decreases at the lowest elevations
(5,000 feet msl) on both sides of the Continental Divide
from Table 14, it is observed that decreases occur in the
average precipitation of about 56% from the -16°C to -20°C
category to the coldest category. This appears to be in
agreement with the decreases in potential condensate over
this temperature range.
At the highest elevations, the decreases in maximum
precipitation amounts from the -16°C to -20°C category to
the coldest category across both profiles is about half
as great percentage-wise as would be anticipated from the
decreasing potential condensate. A possible explanation
for this may be that the utilization of potential conden­sate
to produce precipitation from these orographic type
clouds is not as efficient at temperatures warmer than the
-21°C to -25°C category. Although there is more potential
condensate at the warmer temperatures, there may not be
sufficient ice crystals to utilize the available moisture
in the time interval available. The lack of sufficient
ice nuclei and crystals in the Central Colorado Rockies at
the temperatures warmer than - 21°C has been ve.rified by
Grant et al. in the Climax Project (Bureau of Reclamation
Report 1969).
56
The higher elevation decrease in precipitation for
the warm temperature cases does not continue down to the
lowest elevations. The storm clouds producing precipita­tion
over the central Colorado Rockies were found to have
tops at about 16,000 - 21,000 feet msl (Furman, 1966). The
crystals falling through these storm clouds to the lowest
elevations have about twice the saturated environment in
that they may continue to grow by diffusion and accretion
when compared to the crystals falling on the mountain
passes. Consequently, the crystals reaching the lowest
elevations appear to efficiently utilize the potential
condensate that is available for the additional crystal
growth. In addition, the largest portion of the lower
elevation precipitation results from general storms over
the Colorado mountains rather than from orographic lifting.
This may indicate that the higher level cloud decks above
25,000 feet which are present during general storms are
providing a nearly sufficient concentration of ice crystals
for more efficient removal of condensate. These high level
cloud decks are not present during most of the orographic
precipitation events.
The "special episodes" category that is included in
Tables 6, 7, 10, 11, 14 and 15 includes precipitation
events that were accompanied with wind direction or
velocity changes during the precipitation period of greater
than 30° or 5 mps, respectively or temperature changes
greater than 3°C. These conditions most frequently
57
occurred with the passage of a sharp short wave trough at
the 500 rob level. Also included were precipitation
episodes for which no exact timing could be obtained on the
precipitation within the 24-hour period. Consequently,
the "special episodes" category includes events with a
random sample of wind directions and speeds and temperatures.
This category relates best with those for westerly winds,
moderate wind speeds (16-25 mps), and moderate temperatures
(-26° to -30°C). These conditions are approximately the
"average" conditions that would result if all the meteoro-logical
parameters of the cases studied were averaged to-gether.
The "special episodes" category somewhat represents
an averaged trough passage. The southwest and northwest
winds average out to be in a westerly direction and the
warm pre-trough and cold post-trough temperatures average
out to be moderate temperatures.
Analysis of the Orographic Precipitation Profile Across the
Colorado Rockies
From an analysis of the west to east average precipi-tation
for the 256 cases studied, an approximation of the
orographic effect of the Colorado Rockies can be made. If
the average west and east 5,000 foot precipitation average
of .05 inches per 24-hour precipitation event is the
result of the synoptic convergence of the storm system,
then the additional precipitation above this lower eleva-tion
average which was observed up to the highest eleva-tions
has been considered as the resultant orographically
induced precipitation.
58
For the 256 cases studied from Table 5, it was stated
that 4.74 times as much precipitation is observed at the
highest precipitation sites in the mountains as compared
to the average amount observed at the lowest precipitation
sites at the foot of the mountains and on the high plains.
This orographic average reaches its maximum of 8.15
times for westerly winds greater than 25 mps and tempera-tures
colder than -30°C. It reaches its minimum of 2.53
times for southerly or northerly (parallel to the mountain
ridge) winds less than 7 mps and temperatures between -l6°C
If only the western side of the profile is considered
from Tables 6, 10 and 14 to evaluate the orographic effect,
5.9 times as much precipitation is observed at the highest
sites when compared to the lowest elevation sites.
The windward side orographic effect reaches a maximum
of 9.5 times for northwesterly winds greater than 25 mps
and temperatures colder than -30°C. This effect is at a
minimum of 3.5 for southerly winds less than 7 mps and
temperatures warmer than -16°C.
Synoptic weather Patterns Associated with Specific Broad­Scale
Precipitation Distribution
Studies of two extreme broadscale precipitation
patterns over the Rockies and two extreme local precipita-tion
profiles over Vail Pass have been made in an attempt
to identify the synoptic weather patterns that most
significantly influence these distributions of precipitation
59
with elevation. In selecting the extreme cases for the two
studies, the influencing parameters on the distributions
are expected to reach their maximum differences and should
be easily identifiable.
As a first approach in studying the broadscale
precipitation distributions across the Rocky Mountains, an
analysis was made of synoptic conditions giving rise to
relatively high precipitation events at the lower elevations
(5,000-6,000 feet). Relatively small amounts of precipi­tation
occurred for the same synoptic conditions at the
higher elevations (above 9,000 feet).
Precipitation amounts for the six (6) lowest elevation
stations (average 5,500 feet msl) on the Western Slope were
averaged for each occurrence and compared with the average
precipitation that fell on the 21 snowboard (average 9,500
feet msl) sites on both Vail and Fremont Passes. Thirty­three
(33) cases were observed with substantial amounts of
precipitation (.06 inches per 24 hours or more) at the
lower elevation stations. The average precipitation at the
six low elevation stations for these cases was from .75 to
9.35 times that observed at the mountain sites. These data
are shown in Table 16.
Twenty-nine (29) cases were observed where the average
precipitation across Vail and Fremont Passes was greater
than one-tenth of an inch, while at the same time no pre­cipitation
was reported at any of the six low elevation
stations. These are shown in Table 17. By selecting cases
60
Table 16. Cases Across the East-West Profile with
Relatively Large Low Elevation Precipi-tation
Amounts
Low High
Elevation Elevation
Precipitation Precipitation
Case Date Average Average Ratio
(inches) (inches)
1 5-6-68 .2517 .0269 9.35
2 4-22-68 .1267 .0255 4.97
3 12-27-66 .1925 .0398 4.84
4 12-16-67 .1200 .0333 3.60
5 4-8-61 .4233 .1595 2.65
6 2-10-65 .1283 .0514 2.50
7 3-7-61 .2333 .1076 2.17
8 12-19-64 .0967 .0519 1.86
9 2-13-62 .3483 .1955 1.78
10 4-7-68 .2400 .1410 1.70
11 2-16-62 .1283 .0781 1.64
12 3-28-61 .2183 .1388 1.57
13 3-8-68 .1383 .0890 1. 55
14 5-11-68 .0933 .0657 1.42
15 2-19-61 .1067 .0771 1. 38
16 12-1-67 .1117 .0836 1.34
17 4-10-61 .1967 .1705 1.15
18 4-11-61 .2900 .2600 1.12
19 4-7-61 .0817 .0752 1.09
20 2-26-62 .1700 .1612 1. 05
21 2-15-68 .2017 .1948 1. 04
22 3-22-62 .1283 .1240 1.03
23 4-29-63 .1200 .1167 1.03
24 1-24-67 .0800 .0786 1.02
25 1-28-68 .0633 .0631 1.00
26 1-22-65 .1483 .1531 .97
27 12-16-61 .1300 .1343 .97
28 1-14-62 .1300 .1357 .96
29 5-10-66 .2017 .2114 .95
30 2-10-62 .1067 .1200 .89
31 4-26-67 .1000 .1310 .76
32 3-18-63 .1140 .1512 .75
33 2-22-68 .2575 .3448 .75
61
Table 17 • Cases Across the East-West Profile With Only
High Elevation Precipitation
Low High
Elevation Elevation
Average Average
Case Date Precipitation Precipitation
1 1-30-65 0 .6188
2 4-8-62 0 .6164
3 5-8-67 0 .4700
4 4-23-66 0 .3407
5 2-24-67 0 .2560
6 2-12-67 0 .2412
7 4-5-61 0 .2167
8 3-18-65 0 .1981
9 1-1-61 0 .1871
10 2-23-67 0 .1862
11 2-25-66 0 .1814
12 2-25-63 0 .1774
13 2-14-67 0 .1757
14 1-4-67 0 .1624
15 4-27-66 0 .1602
16 4-10-67 0 .1567
17 3-22-68 0 .1531
18 12-6-67 0 .1521
19 11-3-67 0 .1490
20 4-7-62 0 .1467
21 12-12-65 0 .1445
22 12-21-62 0 .1410
23 11-27-66 0 .1362
24 4-27-61 0 .1352
25 2-17-67 0 .1281
26 1-6-62 0 .1219
27 2-21-66 0 .1131
28 1-22-63 0 .1124
29 5-18-68 0 .1081
62
where no precipitation was observed at the lowest elevations,
the dynamic lifting component of the synoptic storm can be
assumed to be near zero. The high elevation precipitation
is assumed to be that resulting only from the orographic
lifting of the air over the mountain range.
Meteorological parameters at 700 and 500 rob have been
studied for these two extreme cases. Grand Junction radio­sonde
data was used as representative of the lower eleva­tion
Western Slope stations. Denver data was used to
represent the Vail-Fremont area rather than the interpolated
meteorological data used in the previous analysis. Denver
is about 70 miles (straight line distance) from the Vail­Fremont
area. The Denver data should more clearly show
any distinct differences in the upper air parameters near
the top and on the east side of the Rocky Mountains as
compared to those on the lower Western Slope.
For the thirty-three (33) cases where more precipita­tion
fell at the lower elevation stations relative to the
higher elevation stations, an upper-level trough, evident
at 700 and 500 rob, was located west of the Continental
Divide. The main surface storm systems and precipitation
areas associated with the upper level trough passed over
Western Colorado in a northeasterly direction thus pro­ducing
lesser amounts of precipitation over the Central
Colorado mountain ranges. Meteorological parameters common
with this synoptic situation are evident in Appendix II-a.
Higher relative humidities (about 10% average), colder
63
temperatures (about 1.SoC average) and west-southwest winds
(260°) persist over the lower elevation stations relative
to the conditions existing over the Vail-Fremont area in
about 65% of the cases studied.
An analysis of the synoptic surface maps during each
of these storms generally verified the presence of a surface
low pressure center in Western Colorado. The air flow into
the low pressure center was usually not strong enough to
produce significant precipitation along the eastern slopes
that would reach up to the Continental Divide or into the
Vail-Fremont area.
For the twenty-nine (29) cases where at least .10
inches of precipitation was observed across the Vail-Fremont
area and none observed at the lowest stations, an upper
level trough was evident at 700 and 500 rob east of the
Continental Divide. The main surface systems and precipi-tation
areas associated with the upper level trough moved
into the central mountains and Eastern Colorado from the
Northwest. Again meteorological parameters common with this
synoptic situation are evident in Appendix II-b. The
parameters are more evident at 500 rob than at 700 rob. Only
slightly higher relative humidities (about 6% average),
colder temperatures (about 1.SoC average) and northwesterly
winds (295°) persisted over the Vail-Fremont area relative
to the conditions existing over the lower elevation stations
in about 60% of the cases studied. An analysis of the
synoptic surface maps during each of these storms verified
64
the presence of a surface storm system to the east of
Colorado producing orographic precipitation along the
eastern slopes of the Rockies. The gradient and the oro­graphic
flow generally extended up to and frequently over
the Continental Divide and the Vail-Fremont area.
In comparing the average parcel and lifted stability
indexes, available precipitable water and the 700 and 500
rob heights (Appendix I-a and I-b) for these two extreme
cases, it was found that the storms producing the greater
relative precipi tation at lower elevations were 1.5 °c more
unstable, had .25 gm/kg more precipitable water and had
both 700 and 500 rob heights averaging 40 meters lower than
those cases with greater high elevation precipitation.
An analysis of the vorticity fields and movements was
made for storm systems producing these two extreme precipi­tation
patterns across the Rocky Mountains. A limited
amount of vorticity data readily available for analysis
produced thirty-nine (39) precipitation days exhibiting
the two extreme precipitation patterns. Most of these
precipitation days were not part of the original set of
data used in the preceeding analysis. Twenty-two (22)
of the total cases were for episodes with relatively
larger precipitation occurrences at the lower elevations
and the remaining seventeen (17) were for the situations
where significant precipitation was observed only at the
higher elevations.
65
The cases with higher relative precipitation at the
lower elevations were generally accompanied with vorticity
trough or closed centers to the west or southwest or
Colorado with a central absolute value of 15 sec-lor
-1
greater (average 17 sec ). The vorticity trough or
closed center was observed to pass across Colorado from
the southwest or west and move to the, northeast or east
passing by the Vail-Fremont area far to the north or south.
The cases with precipitation observed only at the
highest elevations were generally associated with absolute
vorticity troughs or closed centers of less than 15 sec-1
-1
(average 12 sec ) located north of Colorado moving in a
southeasterly direction or with weak absolute troughs or
closed centers (values from 8 - 13 sec-1) moving across
Colorado in an easterly direction.
In analyzing the synoptic meteorological parameters
giving rise to the broadscale variations in precipitation
distributions, only general synoptic conditions could be
identified and consistent trends in meteorological
parameters could be observed. These results may be useful
in making estimates of the distribution by elevation of
mountain precipitation.
Synoptic weather Patterns Associated with Specific Local
Precipitation Patterns Over Vail Pass
An analysis of two extreme precipitation distributions
across Vail Pass was made to identify the synoptic param­eters
that 'most significantly affected these extreme local
66
distributions. Similarities in the synoptic conditions
that produced relatively large amounts of precipitation on
the western side of Vail Pass while little or no precipita­tion
was recorded on the eastern side of Vail Pass for the
same elevation range and the reverse situation were studied.
Average precipitation amounts for the western side of
Vail Pass were obtained by averaging six snowboard sites
between elevations of 9,200 feet, and 10,400 feet and for
each occurrence compared with the average of four snow­board
sites on the eastern side of Vail Pass at the same
elevation range.
Twenty-two (22) cases were identified where the
average precipitation on the western side of Vail Pass was
2.5 times greater than was observed on the eastern side.
Only seven (7) cases were available for analysis where
greater than twice as much precipitation was observed on
the eastern side of Vail Pass as that observed on the
western side. These twenty-nine (29) cases constituted
the greatest extremes of precipitation distribution over
Vail Pass.
In analyzing the interpolated upper air meteorological
parameters, it was noted that nearly identical parameters
could be observed for each of the two different precipita­tion
patterns. This was interpreted to indicate that it
is extremely difficult to use synoptic scale observations
to evaluate precipitation events that are occurring over
a local area.
-
67
An analysis was made of the surface synoptic weather
patterns that existed over the Vail Pass area during each
of these two opposite precipitation patterns.
For the cases where greater amounts of precipitation
occurred on the western side of Vail Pass, the following
were observed:
1. The surface cyclone was generally passing from the
west to the north or north-east of the study area
and was causing a circulation resulting in south­west
to northwest lower level winds over Vail Pass.
The resulting precipitation appeared orographic in
nature.
2. Pressure rises were usually observed moving into
Western Colorado from the west except when a
second system was following shortly behind the
first.
3. Temperatures on the western side of the Continental
Divide were usually colder than those observed
on the east.
For the cases where greater amounts of precipitation
occurred on the eastern side of Vail Pass, the following
were observed:
1. A cyclone storm system was positioned south or
far to the northeast of the Vail Pass area with
the resulting circulation producing NNE to SE
winds with precipitation occurring from the
eastern slopes westward to the Continental Divide
and mainly on the eastern side of Vail Pass.
2. Pressure rises were observed moving into Colorado
from the north or northeast.
3. Temperatures along the eastern side of the Vail
Pass and to the east of the Continental Divide
were noticeably colder than those observed on the
west side of Vail Pass.
Table 18 shows a breakdown of the occurrences of
conditions observed for each of the two cases studied. In
general, re-occurring trends in synoptic surface weather
68
patterns were observed in studying the factors influencing
the distribution of precipitation in the small scale events.
o
1
3
3o
4
3
o
o
5
o
2o
Number of Cases
Observed with Higher
Precipitation
On Eastern Slope
(7 Total Cases)
4
1
o
3
4
5o
2
4
17
14
10
2
Number of Cases
Observed with Higher
Precipitation
On Western Slope
(22 Total Cases)
North-North­east
East
South-South­east
West-North­west
d.
c.
b.
Conditions
Observed
Table 18. Synoptic Conditions Associated With High
Precipitation Amounts Observed on West
or East Slope of Vail Pass
3. Direction from
Vail Pass of coldest
low elevation sur­face
temperatures
a. North-North-east
b. East
c. South
d. West-North­west
2. Direction from
Vail Pass of
strongest pressure
rises
a.
1. Direction of
Storm Center from
Vail Pass
a. North-North­east
b. East
c. South-South­east
d. West-North­east
e. Over Vail
area
I
II 'I ----
,
69
SUMMARY
This study has focused on obtaining a better
understanding of mountain precipitation and the parameters
that affect its spatial distribution. Precipitation dis­tributions
from 256 and 264 precipitation days for an east-west
and north-south profile respectively, across the
Continental Divide have been analyzed to (1) establish the
mean distribution of precipitation with elevation, to (2)
study the influence of the 500 rob wind direction, velocity,
and temperature on precipitation distributions with ele-vation
and to (3) identify the synoptic conditions producing
variations in the distribution of precipitation across a
north-south oriented mountain range. The results of this
climatological study may be summarized as follows:
1. For an openly exposed mountain barrier, an
average of 4.74 times as much precipitation
is observed near the crest at 10,600 feet msl
than is observed at an average elevation of
5,000 feet msl on either side of the mountain.
2. The ratio of the 5,000 foot precipitation
amounts on the western slopes to the maximum
amount recorded near the openly exposed ridge
line increases from 1:3.0 to 1:11.3 as the
500 rob wind direction shifts from a near
parallel to an orientation perpendicular to
the ridge. The same ratio increases from
1:2.3 to 1:7.5 for a sheltered mountain pass
which is oriented parallel to the main
mountain range.
3. The ratio of the 5,000 foot precipitation
amounts on the western slopes to the maximum
amounts observed near the openly exposed ridge line
increases from 1:4.5 to 1:10.3 when 500 rob
winds increase in velocity from less than 7 mps
to greater than 25 mps. The same ratio increases
from 1:3.2 to 1:8.7 for the same respective
70
wind velocities for the sheltered mountain
ridge parallel to the main mountain range.
4. General decreases of about 55% in average
precipitation amounts are noted at the
5,000 foot elevations as temperatures de­crease
from -20°C to colder than -30°C.
Smaller decreases of about 35% in the
maximum precipitation amounts near the
crest of the profile are noted for the same
range of temperature decrease.
5. For an openly exposed ridge, the influence
that orography has on the distribution of
low to high elevation precipitation amounts
and their resulting ratios reaches its
maximum for 500 rob wind velocities greater
than 25 mps which are oriented prependicular
to the ridge line with 500 rob temperatures
colder than -30°C. The orographic influence
is minimized when 500 rob wind velocities are
less than 15 mps and oriented nearly parallel
to the ridge line with 500 rob temperatures
between -16°C and -20°C.
6. Greater amounts of precipitation are observed
at the lowest elevation windward precipita­tion
sites and on the western slopes relative
to that observed on the ridge line when a
500 rob trough and associated surface cyclonic
storm system are west of the mountain range.
When the 500 rob trough and associated surface
storm system are east of the Continental
Divide, greater amounts of precipitation
relative to the lowest elevation western
slope precipitation sites are observed near
or on the east side of the ridge line.
The results of this study present a detailed picture
of the mountain precipitation patterns that are associated
with variations in meteorological parameters and synoptic
weather patterns. Knowledge of the variations in the
precipitation patterns over a single mountain range can
lead to a better understanding of the total precipitation
picture over mountainous regions.
71
LITERATURE CITED
Elliott, R. D. and R. W. Shaffer, 1962: Relation of
orographic precipitation to orographic lift, air mass
stability and other factors. Technical Paper,
Aerometric Research, Inc.
Finklin, A. I., 1967: Precipitation and runoff character­istics
of the Sacramento River Basin. Master's thesis,
Department of Atmospheric Science, Colorado State
University.
Furman, R. W., 1967: Radar characteristics of wintertime
storms in the Colorado Rockies. Master's thesis,
Department of Atmospheric Science, Colorado State
University.
Grant, L. O. and R. S. Schleusener, 1961: Snowfall and
snowfall accumulation near Climax, Colorado.
Atmospheric Science Technical Paper #17, Colorado
State University.
Grant, L. O.,J. D. Marwitz and C. W. Thompson, 1965:
Application of radar to snow surveying. Proc. 33rd
Western Snow Conference, 42-48.
Grant, L. 0., et al.,1969: An operational adaptation
program for the Colorado River Basin: Interim Report
to Bureau of Reclamation (Contract No. 14-06-D-6467),
Department of Atmospheric Science, Colorado State
University.
Marlatt, W. and H. Riehl, 1963:
the Upper Colorado River.
Research, 68, 6447-6458.
Precipitation regimes over
Journal of Geophysical
Mielke, P. W., Jr., 1967:
with existing ties.
Note on some squared rank tests
Technometrics, 9, 312-314.
Peck, E. L. and M. J. Brown, 1962: An approach to the
development of isohyetal maps for mountainous areas.
Journal of Geophysical Research, 67, 681-694.
Williams, P., Jr. and E. L. Peck, 1962: Terrain influences
on precipitation in the Intermountain West as related
to synoptic situations. Journal of Applied Meteorology,
1, 343-347.
72
APPENDIX I
73
APPENDIX I
EXAMPLE OF AN INTERPOLATED PROFILE ACROSS A
NORTH-SOUTH ORIENTED MOUNTAIN
By using the precipitation ratios given in Table 5, an
interpolated profile can be calculated across a north-south
oriented mountain similar to Vail Pass. An example of this
is as follows:
Wintertime precipitation values were obtained at three
sites across the profile to be calculated. The profile
extends from 6,000 feet msl on the western side up to the
crest at 10,000 feet msl and down to 7,000 feet msl on the
eastern side.
The observed precipitation amounts at 6,000 and 7,800
feet msl on the Western Slope were 9.70 inches and 22.30
inches respectively. An amount of 10.20 inches was observed
at 7,000 feet msl on the Eastern Slope.
From the precipitation ratios across the east-west
profile (Table 5) the interpolated ratios for the elevation
for which precipitation observations are available include
the following:
Precipitation Observed
Ratios Precipitation
6,000 feet W = 1.325 9.70 inches per
season
7,800 feet W = 2.610 22.30 inches per
season
7,000 feet E = 1. 417 10.20 inches per
season
Total 5.352 Total 42.20 inches
Average 1.784 Average 14.07
74
By dividing the average observed precipitation by the
average precipitation ratio, a mUltiplication factor can be
determined. For this example the multiplication factor is
14.07 inches
1.784 = 7.887 inches. The multiplication of this
factor times the precipitation ratio for each elevation will
give the interpolated values of precipitation for the
elevations within the profile. The complete profile is as
follONs:
Precipi-tation
Mu1tiP Ii cati on
Profile Ratio Factor Profile
6,000 feet W 1.325 Use observed 9.70 inches
Precipi tation
7,000 feet W .850 times 7.887 inches 6.70 inches
8,000 feet W 3.050 times 7.887 inches 24.05 inches
9,000 feet W .300 times 7.887 inches 33.91 inches
10,000 feet W 5.925 times 7.887 inches 46.73 inches
8,500 feet E .775 times 7.887 inches 6.11 inches
7,500 feet E 1. 200 times 7.887 inches 9.46 inches
7,000 feet E 1. 417 Use observed 10.20 inches
Precipi tation
Since the crest of the interpolated mountain barrier
is lONer than 10,500 feet, the Western Slope elevation
average relationship should be used in this and similar
cases as the most representative relationship for the crest.
It is not intended that these relationships or interpolated
profiles be extended above 10,500 feet for the east-west
profi Ie.
An estimated precipitation profile can be calculated
from a single precipitation observation within the elevation
range of the profile. A much better estimate ~f the profile
will be obtained if several precipitation observations are
75
used to derive the mUltiplication factor on which the
estimated profile is based.
76
APPENDIX II
Appendix II-a Meteorological Parameters Associated with Relatively Large Amounts
of Precipitation at the Lower Elevations on the Western Slope
Grand Grand
Denver 700 mb Denver 500 mb Junction 700 mb Junction 500 mb
Date GMT Height Wind Temp RH Height Wind Temp RH Height Wind Temp RH Height Wind Temp RH LSI PSI W
700
5-6-68 12 2957 255/04 +3.8 37 5580 240/14 -17.3 50 2964 240/14 +4.0 33 5590 210/16 -19.3 75 .48 1. 93 2.54
4-22-68 12 3007 360/13 -10.0 94 5538 218/05 -24.5 45 2981 071/06 -7.9 96 5522 175/08 -24.5 53 4.01 4.34 2.62
12-27-66 12 2939 350/07 ~7.0 98 5433 223/07 -27.4 32 2916 038/05 -12.4 98 5409 347/04 -29.5 80 8.14 7.04 1. 66
12-16-67 12 2945 200/22 +1.6 53 5578 204/30 -16.8 70 2937 215/09 -6.0 97 5500 183/29 -22.5 71 4.26 4.00 3.20
4-8-61 00 2941 212/10 -4.2 93 5539 237/18 -19.1 70 2034 272/11 -3.2 95 5524 251/15 -19.1 77 4.78 4.38 3.82
2-10-65 12 2921 217/03 -14.3 88 5414 227/11 -28.4 49 2887 130/04 -9.8 80 5394 214/10 -29.0 78 5.98 5.32 .170
3-7-61 12 2914 332/20 -9.0 78 5431 006/09 -26.2 76 2982 328/10 -12.7 65 5459 321/12 -31. 9 60 5.23 4.58 1. 76
12-19-64 12 2978 268/14 -4.7 43 5538 295/22 -21. 9 51 3036 251/15 -8.7 95 5572 270/25 -22.8 48 4.65 4.76 2.23
2-13-62 00 3056 237/10 +6.6 36 5720 226/35 -11.1 48 3058 204/13 +0.5 71 5673 228/24 -16.0 89 5.04 4.87 3.35
4-7-68 00 2919 267/06 +1. 4 38 5510 236/05 -20.0 64 2960 319/13 -6.5 95 5508 234/06 -23.0 73 2.85 3.10 2.59
2-16-62 12 3027 176/03 -0.3 41 5621 237/19 -20.0 33 3026 212/15 -2.8 76 5604 228/20 -21.1 81 .72 2.28 2.64
3-28-61 00 2948 224/02 -6.0 81 5498 228/08 -24.3 73 2943 284/04 -7.1 85 5480 259/13 -25.2 60 .66 1. 59 -2.86
3-8-68 00 2953 320/06 -1. 2 50 5535 295/04 -20.1 62 2993 251/13 -5.7 100 5559 261/12 -23.7 79 1.25 2.14 2.97
5-11-68 00 3066 030/10 -1.3 92 5660 040/08 -19.1 31 3059 065/03 +3.0 43 5690 055/06 -16.9 54 3.59 4.70 2.56
2-19-61 12 3024 243/04 ~O.O 27 5533 285/10 -26.6 19 3034 036/04 ~0.9 31 5539 011/08 -25.3 18 2.45 2.49 1. 81
12-1-67 12 2970 248/16 -0.2 33 5571 231/29 -17.7 19 2954 212/16 -5.4 64 5499 222/24 -24.7 84 2.99 4.14 2.02
4-10-61 12 2969 183/04 -4.8 74 5538 266/20 -21. 9 83 2955 238/11 -2.5 66 5528 250/19 -21.6 71 3.20 3.05 2.88
4-11-61 00 2930 221/05 -2.8 96 5511 256/09 -21. 2 70 2953 012/10 -7.1 91 5503 243/13 -22.3 81 .27 1. 38 3.82
4-7-61 12 3034 197/08 ~0.3 87 5607 268/18 -20.3 34 2997 178/08 -2.5 78 5583 208/15 -18.9 81 8.81 7.74 2.65
2-26-62 00 2879 346/02 -17.8 85 5381 243/17 -27.6 79 2883 246/15 -10.2 58 5379 218/22 -29.3 71 11. 34 8.95 1. 22
2-15-68 00 2990 308/08 -3.4 48 5549 305/11 -23.9 53 3019 261/07 -5.9 83 5564 248/12 -24.4 67 3.27 3.52 1. 95
3-22-62 00 2959 031/03 -2.4 48 5508 274/12 -24.9 48 2974 272/09 -5.0 56 5516 266/14 -25.9 50 -1.27 .58 2.18
4-29-63 00 3052 299/12 -2.8 72 5618 300/17 -23.0 67 3089 249/05 -2.7 71 5669 332/17 -19.4 60 .74 1. 82 3.15
1-24-67 12 2931 285/14 -5.0 39 5480 256/02 -23.6 95 2990 265/09 -10.8 79 5500 297/25 -23.4 36 6.66 5.72 1. 61
1-28-68 12 2963 225/22 -0.9 37 5561 234/31 -19.9 26 2962 195/19 -4.5 95 5529 225/36 -22.9 88 1. 68 3.18 2.54
1-22-65 00 2983 141/01 -2.2 53 5549 245/11 -21. 6 79 2991 247/06 -5.4 90 5554 242/10 -21. 5 79 2.71 2.70 2.67
12-16-61 12 2932 284/05 -6.5 59 5486 233/17 -23.4 50 2955 243/08 -8.6 90 5480 230/15 -27.6 76 4.86 4.57 2.09
1-14-62 00 2897 050/05 -16.0 82 5377 264/12 -29.9 68 2889 234/06 -13.0 82 5377 235/13 -30.0 77 8.03 6.60 1. 39
5-10-66 12 3020 242/10 +6.0 45 5672 225/24 -14.0 26 3028 225/09 +1.3 45 5625 205/32 -20.0 50 4.72 5.85 2.64
2-10-62 00 3069 268/09 +0.3 47 5682 295/26 -15.9 69 3086 242/17 -2.0 89 5694 281/26 -16.4 70 6.56 7.82 3.04
4-26-67 00 3008 305/12 -4.8 63 5562 297/09 -23.9 42 3051 345/04 -4.1 35 5600 279/11 -25.0 25 -.16 1. 62 2.03
3-18-63 12 2923 184/10 -2.6 64 5494 210/09 -21. 8 33 2934 246/08 -9.0 80 5444 203/07 -27.7 33 -1.19 1. 04 2.74
2-22-68 12 2926 323/08 -4.6 86 5497 336/17 -21. 4 77 2966 332/19 -6.3 96 5528 339/13 -22.7 80 3.01 3.01 3.31
Average 2971 270/09 -4.4 63 5539 262/15 -21. 8 54 2981 274/10 -5.8 76 5533 257/16 -23.4 66 3.65 3.96 2.49
GMT - Greenwich Meridian Time
LSI - e e
e 700 mb e 500 mb
PSI - is the difference of the 500 mb temperature and the temperature of a parcel lifted adiabatic1y from 700 mb to
500 mb.
W700 is the liquid water content of a parcel at 700 mb in grams per kilogram.
....,J
....,J
Appendix II-b Meteorological Parameters Associated with Significant Precipita­tion
Occurrences at Only the Highest Elevations
Grand Grand
Denver 700 mb Denver 500 mb Junction 700 mb Junction 500 rob
I I I
Date GMT Height Wind Temp RH Height Wind Temp RH Heiqht Wind Temp RH IHe,1' ghtI Wind Temp RH LSI PSI W700
1-30-65 00 2958 304/20 0.0 51 5550 312/36 -20.0 61 3038 299/11 -2.0 69 5634 330/28 -17.5 61 2.23 3.65 2.99
4-8-62 00 2957 305/10 -5.1 94 5525 307/34 -22.4 80 3016 296/21 +1. 5 48 5614 309/33 -19.5 69 1. 44 2.00 3.20
5-8-67 12 3106 307/14 +5.2 42 5777 316/24 -10.4 38 3140 321/05 +3.6 43 5812 301/17 -10.5 33 6.04 7.01 3.27
4-23-66 00 3051 052/03 -3.6 88 5636 225/10 -19.1 59 3052 316/04 -1. 3 50 5629 217/08 -21.1 50 3.20 3.82 3.15
2-24-67 00 3026 300/18 -7.2 59 5563 315/28 -24.2 57 3075 269/09 -3.3 35 5644 318/26 -19.5 36 -11. 00 -5.02 3.90
2-12-67 12 3055 328/12 -5.2 32 5621 307/21 -22.2 25 3114 280/13 -6.2 39 5680 302/23 -21. 6 74 8.52 7.71 1. 22
4-5-61 00 3035 300/03 -4.5 89 5635 268/35 -17.5 83 3044 316/13 +3.1 54 5671 264/18 -15.9 80 6.87 5.26 3.63
3-18-65 00 2932 316/06 -23.9 74 5409 257/35 -29.3 53 2955 257/14 -8.2 72 5470 292/33 -26.2 51 12.11 9.95 1.14
1-1-61 12 2968 321/16 -12.3 38 5442 265/29 -31. 0 44 2995 297/12 f-13.5 50 5472 285/22 -27.6 19 6.53 5.96 .90
2-23-67 12 3004 303/11 -9.6 66 5515 310/28 -26.5 60 3060 297/08 -7.3 39 5591 313/25 -24.2 88 6.54 5.47 1.52
2-25-66 12 3010 173/08 -2.3 20 5576 190/10 -21. 6 19 3014 197/09 -7.4 61 5550 205/11 -24.5 78 4.66 5.03 1. 29
2-25-63 12 3063 325/06 -9.5 83 5571 339/22 -27.4 19 3089 326/08 -4.2 28 5633 336/26 -25.2 40 2.47 3.38 1. 67
2-14-67 12 2970 t32/13 +1.4 36 5604 266/13 -15.2 23 3007 228/17 +0.4 43 5602 257/30 -19.4 95 4.89 5.48 2.28
1-4-67 00 3004 306/17 -4.3 38 5579 320/33 -20.7 43 3072 295/10 -7.7 89 5636 317/27 -20.3 48 8.16 7.36 1. 91
4-27-66 12 2951 230/06 -5.4 96 5546 233/33 -18.9 98 3028 327/11 -l0.9 35 5551 277/41 -18.1 18 13.89 11. 09 1.82
4-10-67 00 3070 161/12 -2.2 90 5660 234/17 -19.1 20 3067 265/02 -3.0 49 5627 231/14 -23.2 67 2.27 2.75 3.35
3-22-68 12 3048 300/09 -11.2 77 5554 330/39 -22.9 42 3106 327/11 -7.0 49 5680 350/29 -17.8 22 12.77 10.99 1.66
12-6-67 12 2982 236/07 -7.5 58 5492 270/29 -28.4 31 3036 274/14 -10.8 90 5556 285/35 -23.8 34 2.69 3.45 2.04
11-3-67 00 3044 355/07 -15.0 94 5509 302/15 -29.5 60 3046 316/08 -9.2 54 5553 331/32 -22.5 21 7.13 6.53 1.69
4-7-62 12 3012 270/07 -0.3 47 5611 320/18 -19.0 52 3081 285/15 -0.9 38 5674 316/25 -19.3 64 5.21 5.03 2.23
12-12-65 00 2986 323/08 -2.7 69 5552 330/21 -20.6 68 3025 359/02 -5.1 86 5591 311/12 -20.8 45 2.71 3.39 3.25
12-21-62 00 3087 316/06 -7.0 73 5607 311/13 -25.0 41 3108 340/09 -7.4 70 5648 354/22 -22.7 18 3.52 4.35 2.32
11-27-66 00 3011 303/13 -6.9 42 5539 281/26 -25.9 89 3044 282/06 -8.3 77 5562 326/26 -24.7 30 5.76 5.27 1. 73
4-27-61 12 3059 289/10 -4.7 54 5619 305/22 -22.8 18 3088 283/07 -2.3 40 5657 292/17 -21. 0 18 3.37 4.78 1. 94
2-17-67 12 2949 295/14 -12.7 40 5412 305/32 -33.1 62 2993 264/13 -11. 8 58 5500 298/28 -26.5 43 6.31 5.89 1. 02
1-6-62 12 3009 325/16 -2.3 46 5605 335/25 -18.3 80 3087 314/16 -3.6 41 5683 341/44 -17.0 76 11. 01 8.07 1. 92
2-21-66 00 3078 324/11 -6.0 47 5600 300/13 -28.0 ?q 3100 345/03 -8.2 54 5614 346/08 -27.7 34 .94 1. 72 1. 61
1-22-63 12 2984 305/19 -5.9 47 5507 280/29 -27.1 47 3025 264/05 -4.9 65 5590 292/32 -19.9 17 3.56 4.22 1. 98
5-18-68 12 3068 315/08 -3.9 89 5650 300/23 -20.9 53 3086 305/02 +0.8 51 5700 310/10 -16.5 35 2.59 3.83 3.30
Average 3016 296/11 -6.0 61 5568 291/25 -23.0 52 3055 295/10 -5.0 54 5615 300/24 -21. 2 47 5.05 5.12 2.20
GMT - Greenwich Meridian Time
LSI - e e
e 700 mb e 500 mb
PSI - is the difference of the 500 mb temperature and the temperature of a parcel lifted adiabaticly from 700 mb to
500 mb.
W700 -is the liquid water content of a parcel of 700 mb in grams per kilogram.
....,J
ex>

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. .
THE INFLUENCE OF METEOROLOGICAL PARAMETERS
ON THE DISTRIBUTION OF PRECIPITATION
ACROSS CENTRAL COLORADO MOUNTAINS
by
Lawrence M. Hjermstad
This report was prepared with support from
the National Science Foundation
Grant No. GA-11574
Principal Investigator~ Lewis O. Grant
Department of Atmospheric Science
Colorado State University
Fort Collins~ Colorado
May 1970
Atmospheric Science Paper 163
ABSTRACT OF THESIS
This paper presents the results of an investigation of
meteorological factors causing variations in the distribu­tion
of mountain precipitation with respect to elevation.
Precipitation over both north-south and east-west ridges
along the Continental Divide in the Central Colorado Rockies
has been analyzed to identify the local and general contri­butions
of topography to orographic precipitation.
Across an east-west profile over the Central Colorado
Rockies there is an average of 5.83 times as much precipita­tion
observed at the crest (10,600 feet msl) than at an
average western slope base (5,000 feet msl). All of the
significant increase in west slope precipitation with
respect to increased elevation occurs between 7,000-10,600
feet msl (Avg. 7.49 inches per 1,000 feet).
A maximum precipitation ratio of 1:9.5 (base to crest)
occurs when the 500 rob conditions are west to northwest
airflow greater than 25 mps and temperatures colder than
-30°C. A minimum Frecipitation ratio of 1:3.5 (base to
crest) occurs when the 500 mh airflow is parallel to the
ridge with velocities less than 15 mps and temperatures
warmer than -20°C.
An upper-level low pressure trough and associated
surface cyclonic storm systems are generally located on the
western side of the Continental Divide when relatively
larger precipitation observed at the lower
elevations on the western slope. The upper-level trough
and associated surface storm system are generally located
on the eastern side of the Continental Divide when relatively
high precipitation amounts are observed near the crest of
the ridge (10,600 feet msl) or on the eastern slopes.
Lawrence M. Hjermstad
Department of Atmospheric Science
Colorado State University
Fort Collins, Colorado
May 1970
iv
ACKNOWLEDGEMENTS
The author wishes to express his appreciation to
Professor Lewis O. Grant for his helpful suggestions and
encouragement. An expression of appreciation is also due
Dr. Paul W. Mielke, Jr., Dr. Herbert Riehl and Mr. Charles
Chappell for their useful comments and ideas.
This research was sponsored by the National Science
Foundation under Contract GA-11574.
This material is based upon a thesis submitted as
partial fulfillment of the requirements for the Master of
Science Degree at Colorado State University.
v
TABLE OF CONTENTS
LIST OF TABLES • • viii
LIST OF FIGURES x
INTRODUCTION • . . . • . • • •
Background . • •
Statement of Objective .•.
Literature Review of Mountain
Precipitation Studies .•••.
1
1
3
4
DESCRIPTION OF AREA AND CLIMATOLOGY
Central Colorado Rockies . • • • . • .
Fremont Pass .••..•.•....•••
Vail Pass ••..••.•..•
77
9
11
Relationship between Fremont and Vail
Pas se s. • • • • . . . .. • . • . . . • • • •
Climatology of the Vail-Fremont Pass
Are a . . . . . . . . . . . . . . .
Selection of Other East-West Profile Data
Site s . . . . . . . . . . . . . . . . . . .
Selection of Other North-South Profile Data
Sites . • .
12
13
15
18
PROCEDURE . . . . . . . . . . •
Selection of Precipitation Observations from
Fremont and Vail Passes for Analysis •••
22
22
DATA SOURCES AND REDUCTION TECHNIQUES
Precipitation Data Sources
Precipitation Data Reduction •.••
Meteorological Data
29
29
30
32
ANALYSIS . . . . . . . . • • • . . • . • . . •• 34
The Average Distribution of Precipitation
by Elevation Across the Colorado Rockies 34
Analysis of Resulting Preci~itation
Distribution as a Function o~ 500 rob Wind
Direction . . . . . . . . . . . . . . 37
45
Wind
Analysis of Resulting Precipitation
Distributions As a Function of 500 rob
Ve loci ty . . . . . . . . . . . . . .
Analysis of Resulting Precipitation
Distribution as a Function of 500 rob
Tempe r a ture . . • . • . • . • . . . •. 52
Analysis of the Orographic Precipitation
Profile Across the Colorado Rockies • • •• 57
TABLE OF CONTENTS (Continued)
Synoptic weather Patterns Associated
with Specific Broad-Scale Precipitation
Distribution • • • • • • • • • . • . • •• 58
Synoptic weather Patterns Associated
with Specific Local Precipitation
Pat terns Over Vail Pass • • • . • •• 65
SUMMARY . • •
LITERATURE CITED
. . . . . . .
. . . . . . . . . . . . .
69
71
APPENDIX II •
APPENDIX I . . . . . . . . .
. . . . . .
72
76
vii
Table
1
LIST OF TABLES
Climatology of Freemont Pass Area • 14
2 Vail Climatology . . . . . . 14
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
East-West Profile Stations
North-South Profile Stations
Average Precipitation and Ratios Across
the East-West Profile • . . • . • • • •
East-West Profile Precipitation Amounts in
Inches Stratified by 500 mb Wind Direction ••
North-South Profile Precipitation Amounts
in Inches Stratified by 500 rob Wind
Direction . . . . . . . . . . . . . . . .
Summary of Statistical Tests on Data From
Table 6 . . . . . . . . . . . . . . .
Summary of Statistical Tests on Data
From Tables 6 and 7 . • • • . . • . •
East-West Profile Precipitation Amounts in
Inches Stratified by 500 rob Wind Velocity
North-South Profile Precipitation Amounts in
Inches Stratified by 500 rob Wind Velocity
Summary of Statistical Tests on Data from
Table 10 .
Summary of Statistical Tests on Data from
Tables 10 and 11 . • • . . • • . • • . •
East-West Profile Precipitation Amounts in
Inches Stratified by 500 rob Temperature •
North-South Profile Precipitation Amounts in
Inches Stratified by 500 rob Temperature •
Cases Across the East-West Profile with
Relatively Large Low Elevation Precipita-tion
Amoun.ts . . . . . . . .
Cases Across the East-West Profile With
Only High Elevation Precipitation . . •
viii
19
21
35
38
39
42
45
46
47
50
51
53
54
61
62
Table
18
LIST OF TABLES (Continued)
Synoptic Conditions Associated with High
Precipitation Amounts Observed on West
or East Slope of Vail Pass • • • • • • . • 68
Figure
1
2
3
4
5-a
5-b
6-a
6-b
LIST OF FIGURES
Colorado mountain area selected for study •
Vail, Fremont and Hoosier Pass
precipitation networks .•••
East-west profile of mean topography and
mean elevation of observation sites •••
North-south profile of mean topography and
mean elevation of observation sites • •
Frequency distribution of missing snowboard
observations for each site on Fremont Pass.
Frequency distribution of missing snowboard
observations for each site on Vail Pass ••
Frequency distribution of total number of
missing snowboard observations on Fremont
Pass for a precipitation day•••••••.
Frequency distribution of total number of
missing snowboard observations on Vail
Pass for a precipitation day.•...•••
x
8
10
17
20
25
26
27
28
INTRODUCTION
Background
Colorado State University designed and activated a
research program in 1960 to study the feasibility of
increasing winter precipitation through weather modification
in the Central Colorado Rockies. One of the analysis re­quirements
for this study was the development of a high
density precipitation network crossing the Continental
Divide in several places in central Colorado.
Nearly ten years of winter precipitation measurements
in the target areas under both modified and natural precipi­tation
occurrences are now available for study. Although
most of the analysis to date have been concerned with
comparisons of modified and natural precipitation amounts,
this analysis has generated a need to better understand
the precipitation distribution with regard to elevation
under differing precipitation episodes. This is the major
objective of this study.
The precipitation that occurs in the mountainous
regions results from three condensation mechanisms acting
individually or in combination. First, there is the
synoptic horizontal convergence of the air mass into the
low pressure center of the storm which causes ascending
motion~ The precipitation resulting from this mechanism
is, in general, uniformly distributed along the storm
track under non-changing storm conditions.
2
The second precipitation mechanism is the forced
lifting of the air mass caused by the increase in elevation
of the topography. This orographic lifting results in a
second distribution of precipitation that is directly re­lated
to the rate of elevation change of the underlying
topography. The contribution to orographic precipitation
diminishes and reverses as soon as the mountain crest has
been crossed.
The orographic contribution of precipitation may be
divided into a broad-scale lifting caused by the mean ele­vation
change across the mountainous region and a local
orographic effect caused by sharp local elevation change
in the topography. Depending upon the orientation of these
elevation changes to the air flow, these local effects can
playa dominant role in the precipitation distribution
across small segments of the storm track as it moves
across the mountains.
The third contributing mechanism to mountain
precipitation is convection. Convective precipitation is
generally less discernable in winter storms in the Rockies
and is most noticeable in intensifying storm systems or
fast-moving cold fronts with sharp temperature discontinu­ities.
Furman (1967) describes the presence of convective
motions in the form of banded structures embedded in the
orographic snow clouds of Colorado. With only scant radar
coverage and partial histories of recording gage measure­ments
of precipitation for the data analyzed in this paper,
3
no attempt was made to isolate the convective from the
orographic and convergence types of precipitation.
statement of Objective
In order to identify and analyze various precipitation
distributions with elevation, a good understanding of the
parameters influencing natural precipitation distributions
is needed. The objectives of this study are as follows:
1. To describe the average distribution of
precipitation with elevation across the
Colorado Rockies.
2. To determine the changes in the Central
Colorado mountain precipitation profiles
caused by variations in 500 rob meteoro­logical
parameters.
3. To describe the synoptic meteorological
conditions producing general and local
variations in the distribution of pre­cipitation
across the Colorado Rockies.
Once the average distributions of precipitation over
the Colorado Rockies have been identified, a basis for
analyzing or comparing other precipitation distributions
can be established. Precipitation patterns over other
mountainous regions can be compared to those over the
Colorado Rockies in an attempt to study their geographic
variations. Also, natural and modified precipitation
distributions can be compared over the same mountain to
better understand the influence of weather modification on
precipitation patterns.
4
Literature Review of Mountain Precipitation Studies
A study of the precipitation regimes over the Upper
Colorado River Basin (MarIatt and Riehl, 1963) has shown the
role the large and small storm occurrences play in producing
runoff into the Colorado River. Another study of precipita­tion
as a function of elevation was made over the Colorado
and Wyoming mountains using wintertime precipitation
(Finklin, 1967) by interpolating between precipitation
sites. The distances between precipitation sites ranged
from 3 to 36 miles. Elevation changes between compared
sites ranged from 940 to 4200 feet. The annual precipita­tion
increases for the elevation changes that were studied
ranged from 2.14 to 17.36 inches per 1000 feet with an
average change of 6.39 inches per 1000 feet. This large
range of increase in precipitation with elevation resulted
partly from the averaging of precipitation differences over
a large range of distances and from variations in the mean
elevation between the precipitation sites. Also, the
geographical location of some of the compared sites was
such that they were more favorably located with respect to
the climatological storm track through the Rocky Mountains.
A much higher average change of 11.4 inches per 1000
feet was calculated by Finklin (1967) in a study over the
Sierra Nevada Mountains. This higher average rate of
increase in wintertime precipitation with increase in
elevation is mainly due to the higher moisture content of
the warmer low altitude type clouds moving over the Sierras.
5
For the regions between Eagle, Colorado, and Vail Pass
and that between Leadville, Colorado, and Fremont Pass,
Finklin calculated wintertime precipitation increase changes
with elevation of 3.51 inches per 1000 feet and 6.37 inches
per 1000 feet respectively.
In all the above studies, only general inferences were
made that meteorological parameters affected the distribu­tion
of precipitation with increased elevation.
A study in the Southern California mountains by Elliott
and Shaffer (1962) showed some of the meteorological and
dynamic factors that contribute to the formation of oro­graphic
precipitation. Their study also presented a
theoretical distribution of this precipitation across the
mountain range once it had been condensed from the model
cloud system.
Peck and Brown (1962) and others have developed tech­niques
for preparing isohyetal maps for mountain regions.
From these maps correlations were derived between precipita­tion
amounts and physiographic features. From their work
they calculated increases of precipitation with respect to
elevation to be between 2.3 to 4.6 inches per 1000 feet for
several mountainous regions of Utah. No attempt was made
to identify the meteorological parameters contributing to
the terrain's influence in the distribution of precipita­tion
in this study.
Williams and Peck (1962) investigated the terrain
influences on precipitation distribution over the mountain
6
regions of Utah relative to various general synoptic
weather patterns. They found that under upper-level cold
law situations the rate of increase of precipitation with
elevation was approximately 34% of the rate of non-cold
law situations. The rate of precipitation increase with
elevation is .99 inches per 1000 feet for cold laws, and
the rate for non-cold laws is 3.33 inches per 1000 feet.
The studies mentioned have discussed the variation of
mountain precipitation with elevation as a function of a
specific synoptic pattern, physical properties, and dynamics
of the storm system. But, how do the variations in meteorol­ogical
parameters change the pattern of orographic precipi­tation?
This study of the precipitation patterns resulting
from changes in various meterological parameters will lead
to a better understanding of mountain precipitation.
7
DESCRIPTION OF AREA AND CLIMATOLOGY
Central Colorado Rockies
The mountainous region in Colorado considered in this
investigation is shown in Figure 1. The region consists
roughly of the northern two-thirds of Colorado. The western
half of the region is made up of flat-top mountains with
peaks up to approximately 9000 feet msl along the western
edge of the state. The mountain tops gradually become
more rugged and increase in elevation to the Continental
Divide in central Colorado. Here the peaks reach over
14,000 feet msl. The elevation in the eastern half of the
region drops abruptly from the Divide to a relatively flat
elevation around 5000 feet msl.
The mountainous region to the west is interrupted by
three main river drainages: the Yampa River Valley to the
north, the mainstem of the Colorado River Valley in the
central portion, and the, Gunnison River Valley to the south.
Each of these river valleys is oriented roughly west-east
up to the Continental Divide which is generally oriented
north-south. One of the effects resulting from the orienta­tion
of these valleys is the channeling of the airflow to
the Continental Divide where the air rises more abruptly
and the distribution of precipitation with elevation change
becomes most pronounced under favorable synoptic conditions.
Figure 1. Colorado mountain area selected for study.
R
9
Fremont Pass
Fremont Pass lies on the Continental Divide at an
elevation of 11,318 feet. It is approximately 25 miles east
of the Sawatch Range and at the western foot of the Mosquito
and Tenmile Ranges all of which rise to heights over 14,000
feet.
The orientation of the Arkansas River Valley from
Leadville to the pass is approximately 210-230 degrees and
from the pass to Frisco the Tenmile Creek Valley is oriented
approximately 360-10 degrees.
The limited source of the moist air for orographic
precipitation from the southwest comes mainly from air
moving up the Arkansas River Valley south of Leadville or
from moist air spilling over from the Gunnison River Valley.
The source of moisture from the north is mainly from air
moving up the Blue River Valley from the North Park region.
Figure 2 shows the precipitation network from Leadville
across Fremont Pass to Frisco. The network consists of
24 snawboard sites spaced about one mile apart along the
highway and two recording precipitation gages. In addition,
precipitation data taken from a snowboard and recording
gage site at the University of Colorado's High Altitude
Observatory supplements the Colorado State University net­work
data and aids in determining the timing of the
precipitation episodes.
Vail, Fremont and Hoosier Pass precipitation networks
I-' o
r (,
1\ 1\
1\
X13780
(I.
,II.
1\
1\
1\
1\
1\
11~09
1\
1\
1\
Jl. :2738
,II.
1\
1\
1\
7'5
9."~~:::n _ _ _ _ _ _ _ _ _ _ _ . s\o\\v,;
Elevation of Obsef'l°'\Ol'
125 West
Grand Junction
6,000
Distance in Miles
c
4,000
Figure 3. East-west profile of mean topography and mean elevation
of observations sites
- 12,000 Q)
~ 10,000
c: 8,000
.Q-o
>~
w
I
18
The seventeen precipitation sites with recording gages
generally lie in the southwest to northwest and southeast to
northeast sectors from Vail Pass. Table 3 lists the sites,
elevation range, station elevation, their approximate
straight-line distances from the Continental Divide and the
corresponding 24-hour precipitation period used in this
analysis. In designing the profile network in this fashion,
most of the sites are affected by the same weather systems
that produce precipitation on Vail Pass. Usually more than
one station was available for determining the precipitation
at each elevation range as is shown in Table 3.
Selection of Other North-South Profile Data Sites
Five sites with recording precipitation gages were
used in extending the north-south precipitation distribution
analysis beyond Fremont Pass. This analysis was undertaken
mainly to look at the variations in the precipitation dis­tribution
across Fremont Pass under northerly airflow
conditions. The stations were located within 80 miles of
Fremont Pass and each station had precipitation records for
the period analyzed. The stations used in the north-south
profile are indicated in Figure 1 with a small triangle
after the name of the town.
Figure 4 shows a mean north-south profile of the
topography extending from either side of Fremont Pass and
the average elevation of the observing sites. The hori­zontal
distance is again distance from the Fremont Pass
area, and the mean elevation was evaluated from a
Table 3. East-West Profile Stations
19
(Comparative 24-hour Precipitation Period is from 0900-0800
hours. )
38
38
47
39
50
40
7-9
3-5
0-1
3-6
72
90
91
70
60
46
125
81
70
110
35
16-19
Distance to
Continental
Divide in
Miles
6,000
6,285
6,242
6,175
5,960
7,760
7,300
4,855
5,400
8,430
8,500
5,221
5,183
6,497
7,664
Station
Elevation
in
Feet
8,855
8,949-9,125
7,928
7,872-8,176
9,718-10,232
10,488-10,626
10,200- 9,839
5,000
5,000
8,000
8,000
8,500
8,500
5,000
5,000
7,500
7,500
6,000
6,000
6,000
6,000
7,000
7,000
9,000
9,000
6,000
Elevation
Range
in
Feet
10,000
10,500
10,000
Craig
Meeker
Cedaredge
Wilcox Ranch
Grand Junction
Rifle 2 ENG
Station
Eagle
Gunnison
Morrision 1 SW
Aspen
Vail #79,78,77
Woodlake Park 8 NNW
Evergreen 25W
Denver WBAP
Deartrai1
Elk Creek
Lake George 8 SW
Vail #69,68,67
Vail #65,64
Vail #62A,62,61
Crested Butte
Vail #72,71
4,000
~
o
~ ...... _-
47 51 Nath
Hot SUlphur Grand Lake
Springs
15
Frisco
10 °
Sugarloaf Fremont
Reservoir Pass
Distance in Miles
68
Coaldale
\-IIea~ ~\;.';: !!..o~ ..'!f_T..'!P..'!~~~ _ _ _ . -S-la-li"-'" -
lI1eo ! Q'oSe"al\Oo
n Eleva"ot' 0
87 South
Saguache
Figure 4. North-south profile of mean topography and mean elevation
of observation sites
c
8,000
12,000
co
; 6,000
>
Q)
w
+-
~ 10,000
lL.
21
topographi cal map simi lar to the method used in the
profile.
A glance at both Figures 3 and 4 show that the highest
mean elevation in the northern two-thirds of Colorado is
located in the vicinity of Vail and Fremont Passes. This
would suggest that generally orographically generated pre-cipitation
should be evident across this area regardless of
airflow direction.
Table 4 lists the station that represents each
elevation and the corresponding 24-hour precipitation
period. Again, since the sites are within 80 miles of
Fremont Pass, the same weather system that produces precipi-tation
at Fremont Pass will effect the whole network.
Table 4. North-South Profile Stations
Station Distance to Comparative 24
Elevation Fremont Hour Precipi-
Station in Feet Pass in Mi les tation Period
Hot Sulphur Springs
2 SW 7,800 47 0900-0800 hours
Grand Lake 6 SSw 8,300 51 0900-0800 hours
Fremont #23,24 Avg. 9,200 18 0900-0800 hours
Fremont #20,20A Avg. 10,000 12 0900-0800 hours
Fremont #11,12,
13,14 Avg.ll,200 2 0900-0800 hours
Fremont #1,2 and
Sugar load Reser-voir
Avg. 10,100 12 0900-0800 hours
Sagauche 7,700 87 0900-0800 hours
Coaldale 6,900 68 0900-0800 hours
22
PROCEDURE
Selection of Precipitation Observations from Fremont and
Vail Passes for Analysis
Precipitation observations from both the Vail and
Fremont Pass networks from the 1960-61 winter season to the
1967-68 winter season have been used in this study. Precip-itation
measurements across either pass were made during
this period if a quarter inch of snow was observed to have
fallen at any of the sites.
Observations were normally made across Fremont Pass
starting with the site nearest Leadville (Fig. 2) at 0800
hours. The precipitation at all the sites was consecutively
measured across the pass to the site nearest Frisco which
was normally observed around 1100 hours. Observations
across Vail Pass started at 0800 hours with the site near-est
Minturn and then were continued consecutively across
the pass to the last site near the junction of U.S. 6 and
Colorado 91. This last site was normally observed around
1030 hours. The observation technique and data reduction
to equivalent inches of water will be explained in the
Data Sources and Reduction techniques section.
Snowfall occurs very infrequently during the period
of the morning when the snowboards are being observed.
Observations and investigations by Grant et ale (Bureau of
Reclamation Report, 1969) indicate that a strong diurnal
occurrence of no precipitation occurs between 0800 hours
and 1100 hours over the Fremont-Vail area. For this reason
.J
23
the timing for each observation across these two passes
eluded were as follows:
not included.
tion events for the winter seasons from 1960 to 1968 are
The feasibility study operated for varying lengths
The data analysed for this study is only part of a
2. More than 9 precipitation sites were missing in
the network observation.
1. Several days of precipitation were included in one
observation.
analysis purposes. Only 24-hour accumulations were used in
In this study only precipitation data from natural
this analysis since it is the objective of this study to
of time each winter season. Precipitation events were
24-hour portion thereof.
unless otherwise indicated. From 1961 to 1968, 329 natural
has been compared to an average timing of 0900 hours for
on precipitation during a single precipitation episode or
precipitation episodes were recorded at Fremont Pass and
24-hour precipitation episodes were considered for analysis
passes, 264 were used from Fremont Pass for this study and
study.
256 from Vail Pass. Reasons for dropping those not in-complete
set of precipitation data used in the National
evaluate the influence that meteorological parameters have
288 were recorded at Vail. Of these occurrences on both
ity study was operating. Consequently, all the precipita-
Science Foundation sponsored weather modification feasibility
seeded by Colorado State University for a 24-hour period
starting at 0900 hours on a random basis when the feasibil-
24
3. More than 4 consecutive sites were missing in the
observation.
At the time of each original precipitation observation, a
comment was marked on the data sheet as to any pecularities
that were noticed in the snowfall observation such as wind-swept
or melted. On the basis of these remarks, sites that
deviated markedly from its neighboring sites were catego-rized
as missing by data analysts, a frequency distribution
of missing observations was made for each site on each pass
for the network observations used in this analysis and are
listed in Figures 5a and 5b.
Three sites on Fremont Pass were dropped because they
were missing more than 25% of the observations. A frequency
distribution of the number of missing sites for each
observation day for each pass is shown in Figures 6a and 6b.
Figure 5-a Frequency distribution of missing snowboard observations
for each site on Fremont Pass
IV
lT1
(19.3)
51 (18.2)
48
Dropped
( 213)
72
(7.2)
(42)(3.8)(3.8) 19
II 10 10 I I I
(13.3)
35
Dropped
(27.6) Dropped
73 (25.8)
68
(19.7)
52
Snowboard Site Numbers
2 3 4 5 6 7 8 9 II 12 13 14 15 16 17 18 19 20 21 22 23 24
20A
(12.1)(11.7) (11.4) (102)
32 31 30 (8.3) 27 (8.7)
(6.8) (7.6) (6./) (7.6)22 I 23
18 20 16 20
(2.3) I(2;:) r I
264 Episodes t Percent of
Total in Parentheses.
(4.2)
"I
80
70
60 ~
~ :v 50 ~
a.
40 ~
30
20
10
a
"0
L. oo
.n
01 ~
.C- c0
~ if)
-o VI
L. C
(1) .Q
.En -0
:) ~ z (1)
VI .n o
Figure 5-b Frequency distribution of missing snowboard observations
for each site on Vail Pass
tv
0'\
(6.3) (5.5)(5.1)
16 (iP I~ 13
( 14.4)
37
(18.7)
48 (16.0l
41
Snowboard Site Numbers
256 Episodes, Percent of
Toto I in Parentheses
61 62 636465 66 67 686970 71 72 73 74 75 76 77 78 79 80
62A
(7.8) I (10.1)
(6.3) 20 26
16
100
'UI- 90
0
0 80 ~
~
01 0 70
C C 'en CJ) .~ I- 60
~ Q) a. 50 -o II) c 40
I- 0
Q) '- .0 ...... 30 E g
:::J I- 20 Z Q)
II)
~ 0 10
0
100 r 95
I
90
80
(/) 70 30°-60° 150°-190° 200°-240° 250°-290° 300°-340° 350°-20°
6,000 W
Elevation
in feet 4­5
,00° ~'V
7,000 W .020 .016 .047 .045 .024 .016 .022 .034
8,000 W
9,000 W
.119
.083
.080
.115
.123
.178
.129
.182
.132
.186
.098
.121
.093
.142
.122
.172
w
00
10,000 W
10,500
.141
.133
.164
.172
.209
.189
.239
.228
.288
.304
.172
.165
.196
.190
.237
.233
10,000 E .170 .111 .147 .162 .228 .129 .118 .171
8,500 E
7,500 E
6,000 E
5,000 E
.143
.246
.327
.252
.110
.157
.172
.194
.051
.051
.060
.062
.015
.032
.058
.034
.041
.065
.114
.090
.024
.028
.052
.076
.013
.017
.021
.017
.031
.048
.074
.060
Note: There were zero number of cases in the 70°-140° range.
Table 7. North-South Profile Precipitation Amounts in Inches Stratified
by 500 rob Wind Direction
No.
of
Cases 2 4 52 90 82 10 24 264
Range -> 30 0 -60 0 150 0 -190 0 200 0 -240 0 250 0 -290 0 300 0 -340 0 350 0 -20 0 Special Episodes Average
Elevation
7,800 N -t .050 .092 .079 .059 .045 .043 .030 .056
8,300 N .130 .060 .081 .078 .052 .052 .048 .067
9,200 N .008 .102 .123 .136 .138 .066 .093 .126
10,000 N .025 .095 .152 .166 .202 .140 .140 .169
w
11,200 .059 .137 .156 .177 .201 .170 .144 .176 '-0
10,100 S .043 .120 .108 .105 .111 .041 .061 .101
7,700 S .035 .145 .049 .013 .020 .002 .005 .023
6,900 S .140 .017 .036 .014 .004 .006 .010 .016
Note: There were zero number of cases in the 70 0 -140 0 range.
In a
/1 I !
J' /
classl.cal
40
500 rob trough associated with surface
cyclones, the main precipitation associated with the trough
is on the leading edge where southwesterly winds exist. As
the surface cyclone and upper level trough passes an area
and the upper level winds shift direction from southwest to
northwest the precipitation observed at the lower elevations
diminishes. This decrease in precipitation results as the
dynamic influence of the synoptic convergence diminishes.
At higher elevations the orientation of the topography to
the wind's direction still can be influential in the con-tinuation
and intensity of orographic precipitation.
In Table 6 it can be observed that the maximum low
elevation average precipitation is associated with south to
southwest wind directions and it diminishes as the winds
shift to the northwest. At the highest elevations the
average precipitation increases as the winds shift from a
southerly direction to northwesterly direction. This im-plies
that orography influences the generation of mountain
precipitation to a greater extent than the dynamic con-vergence
of the synoptic storm.
The ratio of the 5,000 foot precipitation to the maxi-mum
amount received near or at the ridge line increases from
1:3.0 for south-southwest wind directions to 1:11.3 for
northwest wind directions. The low and high elevation pre-cipitation
amounts used to calculate these two ratios and the
ratios were tested several ways by using the sum of squared
ranks test with adjustments for tied observations (Mielke,
1
1967). A summary of the test results with P-values is
shown in Table 8. TheP-values are the probability of
observing a more extreme test statistic under the null
hypothesis than the one observed. The tests show that the
low elevation precipitation amounts decrease significantly
and the :high elevation amounts increase significantly as the
wind direction shifts from a southwest direction to a north-west
direction. The decrease in low elevation precipitation
amounts appears to be strongly associated with the high
occurrence of zero amounts under a northwesterly 500 rob
wind direction.
As a result of this analysis, it can be stated that
the wind direction plays a major role in the distribution
of precipitation received above 9,000 feet on the central
Colorado Rockies. Average increases up to 50% from a non-orographic
south-southwesterly direction to an orographic
northwesterly direction can be expected above 9,000 feet
msl.
The average precipitation at the low elevations east
of the mountains increases as the wind direction becomes
more orographically oriented with an easterly component.
The southeasterly wind directions are usually associated
with closed cold low patterns evident at 500 rob approaching
or passing to the south of the analysis region. The east-erly
wind direction with this weather pattern normally
extends down to the surface and produces precipitation
along the east side of the Rockies.
J
42
Table 8. Summary of Statistical Tests on Data
From Tab Ie 6
Test
Data Tested Statistic
Occurrence of zero precipita­tion
at low elevations under
S.W. flow vs occurrence of zero
precipitation at low elevations
under N.W. flow 4.85
Maximum high elevation amounts
when zero precipitation at low
elevation for S.W. flow vs
N•w. flow 1. 0 0 3
Ratios of low elevation preci­pitation
to maximum high eleva­tion
precipitation for all non­zero
low elevation cases S.W.
flaw vs N.W. flow 3.833
All low elevation precipita­tion
amounts for S.W. flow
vs N. W. flow 4 • 86 0
All maximum high elevation
precipitation amounts for S.W.
flow vs N.W. flow 2.541
Degrees
of
Freedom P-value
122 .03%
52 23.1%
68 .04%
122 .004%
122 .65%
The northeasterly wind directions are usually
associated with strong surface high pressure systems that
are pushed up along the east side of the Rockies. Oro-graphic
precipitation from the associated easterly wind
flow at the surface results along the east side of the
Continental Divide. This flow does not normally extend
much beyond the Continental Divide.
In general, distinctly different weather pattern is
needed to produce precipitation on the west side of the
Continental Divide as compared to the weather pattern
-
43
necessary for precipi tation on the east side of the Rocky
Mountains.
The orographic effect is less pronounced across the
north-south profile at all elevations. Although the
orientation of Fremont Pass is essentially southwest to
north-northeast, it should be recalled that the pass lies
just to the western side and nearly parallel to the
Mosquito and Tenmile Ranges which rise to over 14,000 feet
ms 1 and are oriented north-south. These ranges should be
more influential in the generation of a more general oro­graphic
precipitation over this area than the local effect
resulting from the orientation of Fremont Pass.
Forty-one precipitation cases common to both Tables
6 and 7 were studied in evaluating the general orographic
precipitation under a southwesterly 500 rob wind direction.
The average 5,000 foot precipitation amount per case from
Tab Ie 6 for these cases was ,.064 inches. The average high­er
elevation maximum was .155 inches for a southwesterly
direction. The resulting low to high elevation ratio was
1:2.4.
Sixty-one precipitation cases were similarly evaluated
from Tables 6 and 7 for a northwesterly 500 rob wind direc­tion.
The resulting ratio of the average 5,000 foot amount
of .031 inches to the average high elevation amount of .228
inches was 1:7.4.
The high and low elevation precipitation amounts and
ratios were tested similar to the data for the east-west
I~I
44
profile using the squared ranks test. A summary of the test
results are shown in Table 9. Again the results show that
both the low elevation precipitation decreases and maximum
high elevation precipitation increases significantly as the
wind direction shifts from southwest to northwest. The
decrease in the low elevation precipitation amounts again
appear to be strongly associated with the high occurrence
of zero amounts under a northwesterly 500 rob wind direction.
For the more general orographic precipitation over the
central Colorado Rockies, it can be stated that wind direc­tion
strongly influences the average increase in precipita­tion
above 9,500 feet msl. This increase approaches about
30% when the wind direction becomes perpendicular to the
surrounding mountain ranges.
The local "lee effect" of precipitation dis tribution
for northerly wind directions is well pronounced over the
south side of Fremont Pass by only traces of precipitation
being recorded at the elevations around 7,000 to 8,000 feet
rosl. Also, the southerly wind direction indicates sizable
increases in precipitation on the south side of the pass
between 8,000 to 10,000 feet msl implying same local effect
of the orientation of the profile.
The effect of the orientation of Fremont Pass appears
to be less significant on the overall distribution patterns
of precipitation than was apparent across Vail Pass. The
general orographic effect of the nearby mountains appears
to be the controlling influence on the precipitation pat­terns
observed over Fremont Pass.
45
wind velocities, from about 50 to 80% from velocities less
than 7 mps to those greater than 26 mps.
.05%
.04%
.60%
1.7%
20.2%
P-value
42
56
100
100
100
Degrees
of
Freedom
Test
Data Tested Statistic
Maximum high elevation
amounts when zero precipi­tation
of low elevations
for S.W. flow vs N.W. flow 1.115
Occurrence of zero precipi­tation
at low elevations for
S.W. flow vs N.W. flow 3.950
Table 9. Summary of Statistical Tests on Data
From Tables 6 and 7
various 500 rob wind velocity ranges for the east to west
All low elevation precipita­tion
amounts forS.W. flow
vs N.W. flow 3.945
and north to south precipitation distributions respectively.
amount per 24-hour precipitation event for an elevation for
Ratio of low elevation
precipitation to maximum high
elevation precipitation for
all non-zero low elevation
cases for S.W. flow vs N.W.
flow 2.640
Not much vari ation is 'noted in the precipi tation
amounts in each velocity group below 7,000 feet in both
tables. At the higher elevations a distinct trend is noted
Analysis of Resulting Precipitation Distributions As a
Function of 500 rob Wind Velocity
Tables 10 and 11 show the average precipitation
All maximum high elevation
precipitation amounts for
S.W. flow vs N.W. flow 2.185
in both tables for higher precipitation amounts with greater
Table 10. East-West Profile Precipitation Amounts in Inches Stratified by
500 rob Wind Velocity
No.
of
Cases 20 77 81 45 33 256
Range-> 0-6mps 7-15 16-25 >25mps Special Episodes Average
Elevation-l-
5,000 W .046 .049 .039 .030 .035 .040
6,000 W .062 .056 .050 .047 .057 .053
7,000 W .041 .029 .037 .046 .022 .034
8,000 W .086 .109 .145 .141 .093 .122 ~
0'\
9,000 W .123 .141 .198 .225 .142 .172
10,000 W .199 .193 .265 .307 .196 .237
10,500 .205 .190 .255 .310 .190 .233
10,000 E .134 .139 .196 .233 .118 .171
8,500 E .071 .037 .030 .029 .013 .031
7,500 E .057 .059 .053 .037 .017 .048
6,000 E .066 .077 .095 .076 .021 .074
5,000 E .072 .072 .075 .041 .017 .060
~
Table 11. North-South Profile Precipitation Amounts in Inches
Stratified by 500 rob Wind Velocity
No.
of
Cases 16 79 94 51 24 264
Range -> 0-6mps 7-15 16-25 >25mps Special Episodes Average
Elevation 4-
7,800 N .037 .042 .065 .078 .030 .056
8,300 N .044 .053 .074 .093 .048 .067
9,200 N .127 .092 .129 .188 .093 .126
10,000 N .146 .128 .173 .244 .140 .169 .c:.
-....J
11,200 .142 .143 .174 .256 .144 .176
10,100 S .072 .079 .099 .166 .061 .101
7,700 S .075 .024 .023 .016 .005 .023
6,900 S .022 .021 .019 .001 .010 .016
;::::==
48
The low elevation values on the north-south profile
would not be representative for comparison with the high
elevation sites due to the relationship previously dis­cussed
with the surrounding mountain ranges. It is interest­ing
to note the nearly consistent ratios of from 1:2.7 to
1:3.9 for all specific wind velocities. This consistency
emphasizes the fact that the orientation of Fremont Pass
plays a minor role in altering the precipitation distribu­tion
across it when compared to the general orography of the
surrounding ranges.
In studying the influence of the 500 rob wind velocities
over the north-south profile, precipitation cases cornmon
to both Tables 10 and 11 were used.
Sixteen cases were evaluated for wind velocities less
than 7 mps. The average 5,000 foot precipitation amount
per case from Table 10 for these cases was .036 inches. The
average higher elevation maximum from Table 11 was .146
inches for these light-wind velocities. The resulting low
to high elevation ratio was 1:4.0.
Thirty-eight precipitation cases were similarly
evaluated from Tables 9 and 10 for wind velocities greater
than 25 mps. The resulting ratio of the average 5,000 foot
amount of .033 inches to the average high elevation amount
of .265 inches was 1:8.0.
The high and low elevation precipitation amounts used
to calculate the two ratios in both east-west and north­south
profiles and the ratios were tested using the squared
49
ranks test. A summary of the test results for both profiles
are shown in Tables 12 and 13, respectively. The test
results for both profiles show that the low elevation
amounts are not changing significantly with respect to the
high elevation amounts as the velocities change from less
than 7 mps to greater than 25 mps. The high elevation
increases are statistically more significant on the north­south
profile than on the east-west profile where the
maximum high elevation amounts increase by about 80% for
velocities greater than 25 mps. The high occurrence of zero
precipitation amounts at the lower elevations when wind
velocities were greater than 25 mps appears to be signifi­cant.
This high occurrence of zero precipitation does
significantly alter the basic distribution of the low
elevation precipitation across the low and high velocity
east-west profiles as indicated by that specific test. The
same is not evident for the north-south profile.
From an analysis of the data in Tables 10 and 11, it
can be seen that wind velocity plays an important role in
the distribution of the precipitation across a mountain
range. The higher the wind velocity the stronger will be
the resulting vertical motion. These stronger vertical
motions through the cloud system over the mountain range
produce condensate at a a more rapid rate which precipi­tates
out over the upper windward side of the mountain.
The slight decrease in precipitation with elevation
between 6,000 and 7,000 feet from Table 10 may be partially
50
Table 12. Summary of Statistical Tests on
Data from Table 10
Test
Data Tested Statistic
Occurrence of zero preci­pitation
at low elevations
for velocities less than
7 mps vs velocities greater
than 2S-mps 3.76
Maximum high elevation
amounts of precipitation
when zero precipitation
at law elevations for
velocities <7 mps vs
velocities >25 mps-- 1.505
Ratios of low elevation
precipitation to maximum
high elevation precipita­tion
for all non-zero low
elevation cases for
velocities <7 mps vs
velocities >25 mps-- .072
All law elevation preci­pitation
amounts for
velocities <7 mps vs
velocities >25 mps-- 2.025
All maximum high elevation
precipitation amounts for
velocities <7 mps vs
velocities >25 mps-- 1.204
Degrees
of
Freedom
63
30
31
63
63
P-value
.04%
10.1%
45.8%
2.4%
17.5%
explained by the selection of precipitation sites at 7,000
feet. They are located in the bottoms of canyons and
consequently no vertical motion would result from either
a northwest or southwest wind. Also, there is no signifi-cant
increase in the topography of the downwind canyon to
make the westerly wind more efficient in producing preci-pitation.
51
Table 13. Summary of Statistical Tests on Data
from Tables 10 and 11.
Test
Data Tested Statistic
Occurrence of zero preci­pation
at low elevation
for velocities <7 mps vs
velocities >25 mps 2.020
Maximum high elevation
amounts of precipitation
when zero precipitation at
low elevations for veloci­ties
<7 mps vs velocities
>25 mps. 1.185
Ratios of low elevation
precipitation to maximum
high elevation precipita­tion
for all non zero
low elevation cases for
velocities <7 mps vs
velocities >25 mps-- .620
All low elevation precipi­tation
amounts for
velocities <7 mps vs
velocities >25 mps-- 1.276
All maximum high elevation
precipitation amounts for
velocities <7 mps vs
velocities >25 mps-- 1.730
Degrees
of
Freedom
52
23
27
52
52
P-value
2.4%
18.9%
33.6%
16.7%
4.6%
The large precipitation increase between 8,000 and
10,600 feet can be explained mainly by the fact that this
change in elevation occurs in a fairly short distance of
about 30 miles while the distances between 5,000 to 8,000
feet occur over longer distances of about 90 miles. The
region with the more rapid rate of increase in elevation
has the more rapid rate of increase of precipitation.
52
The stronger wind velocities have associated with them
a more pronounced "precipitation shadow" on the lee side
of the mountain. This "precipitation shadow" results as the
air rapidly moves across the crest of the ridge and decends
on the lee side at a dry adiabatic rate and warms the
environment. This warming is sufficient to evaporate nearly
all the condensate that was available for precipitation.
Analysis of Resulting Precipitation Distribution as a
Function of 500 rob Temperature
Tables 14 and 15 show the average precipitation amount
per 24-hour precipitation event for various elevations and
500 rob temperature ranges for the east to west and north to
south precipitation distributions respectively.
Little change or even a slight decrease in the maximum
high elevation precipitation is observed in the -16°C to
-25°C category when compared to the -21°C to -25°C category
in both Tables 14 and 15. A decreasing trend in the average
precipitation at the higher elevations with decreasing
temperatures colder than the -21°C to -25°C category can
also be noted. From the -21°C to -25°C category to the
-26°C to -30°C category on both tables the maximum higher
level precipitation decreases by 14% and 21%, respectively.
From the -21°C to -25°C to the category colder than -30°C
the decreases are 33% and 37%, respectively. The four cases
in the warmest category do not constitute a large enough
sample from which any sound conclusions may be drawn.
Table 14. East-West Profile Precipitation Amounts in Inches Stratified by
500 mb Temperature
No.
of
Cases 4 39 93 66 21 33 256
Range -> OOC to -15°C -16°C to -20°C -21°C to -25°C -26°C to -30°C O°C to -15°C -16°C to -20°C -21°C to -25°C -26°C to -30°C